AbstractThe effects of the organometallic compounds Bu2Sn‐D‐(−;)sorbitol, Bu2Sn‐D‐(+)glucose, Bu2Sn‐D‐(−)fructose and Bu2Sn‐D‐(+)glyceraldehyde were tested in vivo on different stages of Ascidian development, larval movement and metamorphosis. Organotin(IV) complexes are organometallic compounds widely used as industrial biocides, antifouling agents and agricultural fungicides and are toxic to a range of organisms. Two‐cell stage embryos, if incubated for one hour in the organotin (IV) solutions, stopped the cleavage, which was restored when they were transferred into normal sea water. The gastrula stage was seriously affected in 10−4mol dm−3 solutions of the above‐mentioned complexes: 85% of the embryos were anomalous neurulae with open neural folds, 5% were twisted larvae. The gastrulae, when incubated for 1 h in 10−5mol dm−3 solutions, developed twisted larvae in ovular envelopes and immobile larvae with twisted tails. Larvae treated with 10−4mol dm−3 and 10−5 mol dm−3 Bu2Sn‐D‐(−)sorbitol, Bu2Sn‐D‐(+)glucose and Bu2Sn‐D‐(+)glyceraldehyde solutions stopped swimming, did not metamorphose and afterwards underwent cytolysis. An initial hyperactivity of circular movements, followed by immobility, was observed in the larvae incubated in Bu2Sn‐D‐(−;)fructose.