In order to obtain basic data on mechanisms controlling starfish development, we examined the effects of the inhibition of RNA synthesis on the development of the starfish,Asterina pectinifera.Embryos were treated with formycin A in three ways: (a) Continuous treatment starting at various stages, (b) treatment for various lengths of time following 1-methyladenine (1-MA) administration and (c) pulse treatment for 2h at various stages. Results indicated that: (1) embryos treated with formycin A from the beginning of 1-MA treatment arrested as early blastulae; (2) treatment with formycin A inhibited rotation, hatching and vegetal plate formation depending on when they were treated; (3) treatment with formycin A from mid- to late-blastula stage inhibited gastrulation; (4) treatment with formycin A from late blastula to early gastrula stage prohibited mesenchyme differentiation, mesenchyme migration, and coelomic pouch formation depending on the time of treatment. Developmental arrest was reversed by the addition of uridine and cytidine.