Nuclear S1 proteins are a group of proteins apparently ubiquitous in vertebrate cell nuclei. They were originally isolated at pH 4.9 from the supernatant of rat liver nuclei mildly digested with DNase I. In the present study, under the conditions identical to those employed for vertebrate cells, we identified two S1 proteins in the starfish Asterina Pectinifera. Their molecular weights are 47,200 and 39,000. This finding suggests widespread occurrence of S1 proteins in eukaryotes and their basic function in the cell nucleus.