…that’s what Kay confronted Michael with early in GFIII. How did Anthony find out Michael had Fredo killed?

The most likely answer is that Anthony figured it out for himself. He was lifted out of the boat after Connie told Fredo and Neri that “Michael wants to take him to Reno now” [emphasis added]. But neither he nor Michael went to Reno. Michael was in the boathouse when the shot rang out, and if he heard it, Anthony could easily have heard it, too, even if he wasn’t in the boathouse. We can assume that Neri came up with some story about Fredo falling overboard and disappearing ‘neath the waves, but Anthony, hearing that shot, would have put two and two together.

Another possibility: Kay told Anthony. Michael would have to explain their mother’s sudden banishment, and the subsequent divorce, to Anthony and Mary. We saw the look of fear on their faces when Connie rushed Kay out of her visitation with them—and she encountered the Dreaded Corleone Stare. I’m guessing that Michael told them that their mother did something terrible—maybe using the same words Kay used to describe her abortion: “Unholy and evil.” The kids would have confronted Kay with that during one of her visitations. And Kay, resenting Michael’s attempt to portray her as the “unholy and evil” parent, would have defensively told the kids that Michael was more “unholy and evil” because he had his own brother killed.

What do you think?

Good question. It could be a little of both. I say this because in another scene in III Mary asks Vincent if Michael killed Fredo, but in a tone of voice that sounds like she wishes it was not the case. Vincent's answer is a bit on the lame side but he does say "no."

So it could be a combination. Anthony knew he didnt go to Reno and could have heard the shot, followed by the clear fiction that Fredo had drowned (I bet there was not much of a search party). Anthony in turn could have shared this with his sister who then asked Kay. Since Kay was not around at the time, however how would she know unless Connie intimated it to her? More likely, if Kay and Anthony, once they were back in Kay's custody asked her about Fredo she could have told them what a monster Michael was and how she believed him capable of killing Fredo.

Good point, dt. Kay couldn't have been on the estate when the shot rang out. But she knew Michael had Carlo killed, and once Fredo's MIA was reported, she'd have known instantly that Michael had him killed.

I like the idea of Anthony figuring things out for himself-maybe helped along with a few nondenial denials from people over the years. I think every family has things people know about but don't discuss openly.

...or, as Michael said to Anthony in that same scene in III: "Every family has bad memories." What he neglected to say was that he was the one who created the bad memories.

If by "bad memories" Anthony was specifically referring to the assassination attempt in their home, then Michael would probably say that those memories were Roth's fault, not his. I agree that that's at best an oversimplification, but people (or organizations, businesses, countries etc.) don't usually accept their fair share of the blame for things--this isn't something so unique to Michael.

in my opinion the most reasonable assumption is that Anthony figured things out for himself.

after the "go to Reno now" call, Anthony probably was taken somewhere else where a few minutes later he was told they wouldn't go to Reno anyway, for whatever reason. I assume that in an old school italian family children that age would just comply with what they were told and not ask why.

then he would've heard some story about uncle Fredo going to work at Florida or Mexico or Brazil or whatever and never seen him again. and then, as his knowledge about his father's business grew bigger, coupled with him overhearing things, he would just put 2 and 2 together.

it's common that family secrets are revealed in a fight or a rough discussion like someone yelling "what are you gonna do? just like you did with Fredo?" to Michael and Anthony overhearing and then as his feelings about his father's business turned rebel - we know he didn't anything with "the family" and really wanted to become an opera singer - he hears some years later that his father and his uncle Fredo had a huge fight back in the days and then "bam! dad's a cold blood, ruthless killer!"

I think the trauma of Vito's orange peel smile left it's lasting scars on the boy.

Anthony, as he got older, would probably have figured it out for himself. And it was HE who probably confided it, or his suspicion of it, to his mother.

The possibility of Kay telling Anthony is highly unlikely, because not only was she not around at the time, and maybe not even aware of Fredo's BETRAYAL, but even if she suspected it would not intentionally expose her son to the thought that his father had would have his own brother killed. It was Kay who had attempted (and failed) to shield her children from this world, so even after banishment would not volunteer such information to her son, for any reason.

Apple

More than that, his having killed his grandfather with insecticide spray was really what was behind his "bad memories" line.

Of course, it happened before Rachael Carson published "Silent Spring," so Anthony got a pass...

I like Apple's assesment of the situation. As Anthony got older he started to put 2 & 2 together and figured out that it doesn't equal 5 And then shared his suspicions with Kay.

And neither one of them shared those suspicions with Mary as they knew how she felt about her father. Granted she asks Vincent in that scene from part III but I think that's mainly due to her hearing rumors. And lets face it people rumors can spread faster than a very contagious disease.