By performing interferometric measurements on the fundamental transverse mode of a planar VCSEL, we have determined the wave front curvature inside the VCSEL cavity. The results show that the (apparent) position of the beam waist is 0–8 &mgr;m below the surface of the top DBR, i.e., up to 25% of the Rayleigh range. From these results we estimate the amount of index and gain guiding in the VCSEL. ©1995 American Institute of Physics.