Single leptoquark production at e+e- and gamma gamma colliders.

We consider single production of leptoquarks (LQ's) at [ital e][sup +][ital e[minus]] and [gamma][gamma] colliders, for two values of the center-of-mass energy: [radical][ital s] =500 GeV and 1 TeV. We find that LQ's which couple within the first generation are observable for LQ masses almost up to the kinematic limit, both at [ital e][sup +][ital e[minus]] and [gamma][gamma] colliders, for an LQ coupling strength equal to [alpha][sub em]. The cross sections for single production of second- and third-generation LQ's at [ital e][sup +][ital e[minus]] colliders are too small to be observable. In [gamma][gamma] collisions, on the other hand, second-generation LQ's with masses much larger than [radical][ital s] /2 can be detected. However, third-generation LQ's can be seen at [gamma][gamma] colliders only for masses at most [similar to] [radical][ital s] /2, making their observation more probable via the pair production mechanism.