The temperature dependence of egg laying was examined in winter-caught Aplysia. Cold-water Aplysia californica and warm-water A. brasiliana were individually housed in the same large aquarium for 16 days at 15 degrees C, and then for 16 days at 20 degrees C. Initially, the majority of the A. californica were not reproductively mature (as determined by injections of atrial gland extracts) whereas all of the A. brasiliana were reproductively mature. When the temperature was increased from 15 to 20 degrees C, both species showed a marked increase in the frequency of egg laying. At both temperatures, A. brasiliana laid eggs more frequently but produced smaller egg masses than A. californica. We conclude that increased egg laying in A. californica was attributable both to facilitation of oogenesis in previously reproductively immature animals and to increased activity of the bag cells which release an egg-laying hormone. Increased egg laying in A. brasiliana was attributable primarily to increased bag cell activity.