The ontogeny of behavioral responses of larvae of the crabs Rhithropanopeus harrisii and Neopanope sayi to rates of change in temperature were analyzed using a video system. A temperature decrease evoked an ascent in both species. The threshold rates of decrease for Stages I and IV zoeae of R. harrisii, and Stage I zoeae of N. sayi, were 0.06, 0.1, and 0.09°C min-1, respectively. Stage IV zoeae of N. sayi were unresponsive to any rate of decrease. Larvae descended upon a temperature increase. For Stages I and IV zoeae of R. harrisii and Stage I of N. sayi the threshold rates of temperature increase were 0.07, 0.24, and 0.18°C min-1, respectively. Stage IV zoeae of N. sayi were again unresponsive. In general, there was an ontogenetic change in responsiveness as Stage IV zoeae of both species were less sensitive than Stage I zoeae. The average absolute amounts of temperature change needed to evoke a response was independent of the rate of change at rates above threshold and ranged from 0.29 to 0.49°C for both species. A consideration of larval sinking rates and ascent speeds, as well as normal environmental temperature gradients, shows that larvae of both species can respond to the rates and amounts of temperature change found in their environments. These responses constitute a negative feedback system that could be used to regulate depth relative to temperature.