Oligopeptides 2a-2d derived from the oostatic decapeptide (TMOF) sequence, H-Tyr-Asp-Pro-Ala-Pro-Pro-Pro-Pro-Pro-Pro-OH (1a) and containing isosteric structures were synthesized and assayed to determine their effect during reproduction in the flesh fly Neobellieria bullata. The N-terminal linear tetra- and pentapeptides 2a, 2b containing the Pro-psi[CH2O]Ala isosteric linkage affect egg development in 80-90% of ovarioles resulting in some resorbed egg chambers, abnormal yolk deposition, the formation of large eggs with irregular yolk granules and proliferation of follicular epithelium. In comparison with their nonisosteric precursors 1b, 1c they exhibit even more accelerated oostatic activity. However, peptides 2c, 2d containing a Pro-psi[CH2S]Ala isosteric linkage are less active.