Three different patterns of population sex structure that implied distinctive patterns of sex expression were observed in isolated populations ofPinctada margaritifera (Bivalvia, Pteridae). A natural population appeared to be typically protandric; a colonizing population on an off-shore platform appeared to be gonochoristic; but an established population on a second platform appeared to have pseudo-protogynic sex change. It is proposed that this latter phenomenon was a tactical response to the stress of oil pollution.