Better understanding of the factors that affect the growth of microalgae used as feed in the aquaculture industry will help to improve cultivation methods. In this study, we investigated the effects of Diacronema lutheri filtrates on the growth of D. lutheri itself under various growth conditions to determine whether allelochemicals were affecting the growth of this dietary alga. Our results indicated that filtrates of the D. lutheri reduced the maximum fluorescence of D. lutheri cells in a concentrationdependent manner under various test conditions. In addition, growth inhibitors that accumulated at high concentrations in the culture medium inhibited the growth of D. lutheri when the cultures were incubated at a high temperature. On the other hand, the inhibitory effect of D. lutheri filtrate on the growth of D. lutheri was a species-specific phenomenon, because growth of D. lutheri was not affected by Nannochloropsis sp. filtrate. Furthermore, the permeate fraction obtained by ultrafiltration significantly inhibited the growth of D. lutheri（P < 0.05), but the retentate fraction did not. These results suggest that lowmolecular- weight substances, with the exception of proteins, are involved in the inhibitory effect of D. lutheri filtrate, and developing a process for removal of growth-inhibitors from culture media will contribute to improving the D. lutheri mass culture methodology at high water temperatures.