Comparison of 2 water samples, one collected from 10 m, the other from the aphotic zone (1000 m) on the Costa Rica Dome in the eastern tropical Pacific Ocean, revealed the presence in both samples of pigmented cells of several diatoms, dinoflagellates and coccoid organisms. Measurements of carbon assimilation rates in temperature-controlled incubators across a light gradlent demonstrated that the assimilation number (mg C [mg Chl a]-' h-') of the 1000 m sample was about 0.8, slmilar to that of the 10 m sample. The ratios of RuBP carboxylase to other carboxylating enzymes were also similar between 10 m and the aphotic zone. However, the initial slope a and the inhlbltion parameter P were considerably higher for the deep sample than for the 10 m sample. Possible mechanisms by which these viable algae reached the aphotic zone are discussed.

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