Nineteen novel N-acetyl-2,2-dialkyloxazolidines (2) and N-acetyl-2alkyl-4,4-dimethyloxazolidines (3) were synthesized from commercially available carbonyl compounds and ethanolamine or 2-amino-2-methyl-I-propanol. Their bioactivity against laboratory-reared mosquitoes was compared in patch tests to known N-acetyl-2-alkyloxazolidines (1) and N,N-diethyl-m-toluamide (deet insect repellent). Isomeric composition measurements by ["ClNMR spectroscopy favoured the Z rotational isomer for samples of 2 (91-96% Z) and the E rotational isomer for samples of 3 (66-71% E), in agreement with molecular mechanics calculations on rotational isomers of model oxazolidines. Samples of 1 were previously shown to exist in solution mostly as the Z isomer (60-70% Z). Within the optimal molecular weight range for these experimental chemicals, the duration of repellency against Aedes aegypti (L.), Anopheles quadrimaculatus Say and Anopheles albimanus Wiedemann generally followed the order: 1 > 2 > deet > 3. Bioassay data are discussed in relation to the equilibrium populations of rotational isomers for substituted N-acetyloxazolidines.