UV irradiation of the vegetal pole of stage 2 X. laevis embryos with doses of 6000-18000 ergs/mm-2 delayed or inhibited cytokinesis in the vegetal hemisphere. Nuclear division continued so that a syncytium was formed, the size and persistence of which depended on the dose. Embryos which received a low UV dose were subsequently able to recover whereas embryos receiving a high dose were unable to gastrulate successfully. The implications for investigations into the role of cytoplasmic determinants are discussed.