SUMMARYThree species of tropical oceanic phytoplankton were isolated from two locations in the eastern Pacific Ocean. Unialgal cultures were maintained in an enriched seawater medium. The effects of temperature, and, in separate experiments, illuminance, on the exponential cell division rates of those algae were investigated. For 2 isolates ofGymnodiniumsp. (probablyG. simplex),maximum growth rates were 1.25 and 1.7 divisions/24 hr, the optimum temperature range was 23–29 C, the compensation illuminance was 35 ft‐c, and the saturation illuminance was 750 ft‐c and above. For a small species ofChaetoceros,the maximum growth rate was 6.0 divisions/24 hr, the optimum temperature range was 23–37 C, the compensation illuminance was 10 ft‐c, and the saturation illuminance was 600 ft‐c. For a smallNannochlorisspecies, the maximum growth rate was 4.5 divisions/ 24 hr, the optimum temperature range was 27–37 C, and the saturation illuminance was 800 ft‐c. Nannochlorisgrew heterotrophically by apparently utilizing organic matter supplied