The effect of 5 silatranes RSi(OCH 2 CH 2 ) 3 N, where R = C 2 H s O; C 6 H 5 ; ClCH 2 and RSi[OCH(CH 3 )CH 2 ] 3 N, where R = C 6 H 5 , ClCH 2 and tetraethoxysilane were studied on the growth, photosynthetic oxygen evolution and dark respiration of the marine centric diatom Cyclotella cryptica. Both silica-free and Si(OH) 4 supplemented media were used. Only 1-ethoxy- silatrane was hydrolyzed by the diatom, giving sufficient quantities of Si(OH) 4 to maintain growth without exogenous Si(OH) 4 . In this case the diatoms multiplied by a factor of 18 within 48 h. With silica in the medium, 1-phenylsilatrane, 1-phenyl-3,7,10-trimethylsilatrane, 1-chlormethylsilatrane and 1-chlorm ethyl-3,7,10-trimethylsilatrane exhibited only slightly inhibitory activity. In contrast, however, tetraethoxysilane (5 · 10 -4 mol/1) inhibited growth almost completely. Respiration was unaffected by all the silatranes tested, but net photosynthetic oxygen evolution was reduced by approx. 50% by 1-chlormethylsilatrane and 1-chlormethyl- 3,7,10-trimethylsilatrane and almost completely by 1-phenylsilatrane and 1-phenyl-3,7,10-tri- methylsilatrane. In the absence of silica as nutrient in the medium, the silatranes hardly affected dark respiration within 24 h. Conversely, the same samples showed no photosynthetic oxygen evolution after 24 h, and 24 h later again a negative O 2 balance was produced in the light. Different silatranes produced significant differences in the final oxygen balance.