The growth of Chlorococcum echinozygotum was previously known to be inhibited by an autoclaved solution of xylose in Bold's modified Bristol's medium. We now report the isolation of 2 inhibitory compounds derived from xylose during autoclaving, and the identification of one of them as xylulose. Two strains of C. echinozygotum were isolated, one of which had physiologically adapted to xylulose. The growth rates of both strains were studied using varying amounts of xylulose. Cell counting studies indicated that the zoospores of the unadapted strain were most severely affected by xylulose inhibition and that the zygospores were most resistant.