A PIC logic 0V output is not equal to a ground?

[HankMcSpank]

I've put a question mark at the end of the Subject heading, but I can kind of answer my own question!

If I physically ground the +ve input to this opamp pin (using a flying lead to connect the pin to ground), the opamp's output inverts vs the input (as it should)....



but if I then change by connecting that same opamp pin but directly to a PIC pin & then set the PIC pin as an output 'LOW' - weird stuff happens (the output from the opamp looks akin to a full wave recified signal - ie where the negative cycle is flipped up)

So, a PIC's 0V ouput isn't ground? (no laughing at the back - I might be stating the obvious to you, but not to me!)

What would be my best option to ground that opamp pin under PIC control? (I've tried a CD4066 with poor results - same thing ....the opamp's output looks full wave rectified!)