I received my samples from TI and realized that instead of the 4558 op amp, I instead ordered the 4559 op amp.  I originally wanted these chips for the MXR Noise Gate that I'm fixing to build.  I looked at the datasheets, and they seem relatively similar to me (same?).  Are these chips interchangeable for the MXR NG?  If so, what might I have to do to the circuit to implement these changes?

Quickie-answer:
check the datasheets to make sure pinout and supply requirements/maximums match, or are within usage environment.
If so: plug it in and hear what it sounds like.

According to the Small Bear site, some people use them in Tubescreamers, so I'd imagine the pinouts are exactly the same.

yeah, the pinouts are the same.  There are some differences in the datasheets, but it seems minimal, and in places that shouldn't make a difference. I hope.  I'm currently working on a possible replacement chip for the lm386.  Its the opa134pa chip with fet input stage.  It wont be an exact replacement, but it has some promising looking specs (I think, at least.  Im not entirely sure what im looking at.)  Im attempting to make it work right now in my Big Daddy mod circuit on breadboard.

From what I've heard, the 4558/9 are from the same line. the 4559 apparently has a tad more gain than the 4558. The 4560 has even more gain than that, and that's why I ordered a bunch of each from NJM  :wink:  I'll let ya know if there's a difference. The pinouts are identical on all 3 btw

Thanx Everyone!  Keep me posted Impaler.