A marine bacterium Pseudoalteromonas sp. strain A25 exhibits strong algicidal activity against diatoms. It could completely lyse the centric diatom Skeletonema costatum (Bacillariophyceae) NIES-324 within 2 days when inoculated at a density of 10(4) cells ml(-1). Analysis of the cellular proteins of strain A25 and its algicidal activity-lacking mutant, A25W1, by two-dimensional (2-D) electrophoresis indicated that a large number of proteins were induced in the stationary phase cells of strain A25. These proteins were not detected in late-logarithmic phase cells of strain A25, nor in either the late-logarithmic or stationary phase cells of the mutant A25W1. Algicidal activity against S. costatum was also detected in cell-free extracts, but only in those from stationary phase cells of strain A25. These results suggest strongly that some of the proteins induced in the stationary phase contribute to the lysis of S. costatum by Pseudoalteromonas sp. strain A25.