So I've been playing the Ruy Lopez as white in a run of recent games and have been running into trouble with this line which frequently occurs at my level of play.

I know 3 . . a6 4. Ba4 is the Morphy Defense.  I also know, I think, that black usually doesn't follow with b5 (could be wrong).  Unfortunately, at my level, they do and it's been causing me problems.

Any suggestions on how to either avoid this situation or to capitalize as white in some way that I just don't see.

Note: this is not a current position in any of my live games, but, rather, something I encountered 4 or so times in the last month.

Thanks -

CMI

There's nothing wrong with an immediate b5, and there's only one move for you to reply with. Where's the problem?

The only way to avoid it is to stop playing the Ruy Lopez, and you can't capitalise because it isn't weak.

Normally b5 isn't normally immediate simply because white isn't really threatening to win a pawn until he supports his own rook.

I know I only have one response.  It was just one of those things were you don't see 4. . b5 in any of the intros. to the Ruy that you see at my level (Idiot's guide to openings, etc.)  Thought there might be something I was missing that better players use in response to make that an unpopular line at higher levels.

Black very often plays b7-b5 in the ruy lopez, just not this early.

The reason is because black has no hurry in playing this move. White is not really threatening to play Bxc6 followed by Nxe5 until hes pawn on e4 is defended. So the general rule of thumb is to wait with b7-b5 until white defends hes e-pawn.
It's better for black to be flexible as possible with this advance.

Some people also claim there is a more concrete problem to using this move-order if black tries to develop normally and transpose to standard lines. Unfortunately I don't know enough about this line to say for sure if it's good or not, but I'll give you the line which supposedly punishes black or using this move-order.

Black usually isn't in a hurry to play b5, because, as someone pointed out, White isn't threatening to take the pawn. While it's true White has only one move, I'm sure the OP wanted to know if there were any new possibilities for White after an early b5. In many games play will transpose back to normal lines, but there may be a chance to mess up Black's pawn structure with a quick a4. The fact that White's bishop is now on the dangerous "Italian" diagonal, there may be tactical chance based on the weakness on f7.

At your level, you could try meeting 4. ... b5 with:

It would be good if I could remember the source of where I learned about "the punishment". I do however have some vague memory of the ideas. I guess it's just not as good as this source made it out to be. I've also heared quite strong players saying this is a good line for white.

It might be instructive to post what I remember anyway.

6. d4 Nxe4 is met with 7. dxe5 which usually isn't a good idea in this kind of situation with the bishop still on a4 since when black plays Nc5 the bishop is forced to be exchanged for one of the knights.
Whites idea now is to meet 7...Nc5 with 8. Bd5. Then put pressure with active development. Supposedly giving white a slight advantage.
 

If, as our IM comrade states, 6.Ng5 is a bad/"stupid" move, why would it be OK for someone of the OP's level to play it?