Abstract Continuous incubation of fertilized sea urchin ( L. pictus and S. purpuratus) eggs in 10 −4 M l-chloro-3-tosylamido-7-amino-2-heptanone (TLCK), a specific inhibitor of trypsin and trypsinlike enzymes, results in a cumulative delay in the time of cell division. Compared with untreated eggs, TLCK-treated eggs require 10% more time to complete the second division and more than twice as much time to reach the prism stage. The TLCK-treated eggs also remain in metaphase longer, at least through the first two divisions. They do not differ morphologically from untreated eggs at the same division stage. The TLCK treatment has little direct effect upon incorporation of thymidine into a DNase-sensitive, TCA-insoluble fraction. Little or no division delay is elicited by l-tosylamido-2-phenylethyl-chloromethyl ketone, iodoacetic acid, soybean trypsin inhibitor, lima bean trypsin inhibitor, ovomucoid trypsin inhibitor and pancreatic trypsin inhibitor. The division delay is not due to inhibition of cortical granule proteases.