The influence of saponin extracts isolated from two different plants: soapwort (Saponaria officinalis L.) and lily-of the-valley (Convallaria maialis L.) on the process of egg laying by females of Colorado potato beetle (Leptinotarsa decemlineata Say) was studied. Colorado beetles were reared on potato leaves treated with saponin extracts as well as on the control – untreated leaves. Experiment was conducted during four weeks. Survival of beetles, number of egg deposits and the number of eggs in each deposit were recorded every day. The presence of both kinds of saponin extract in tested pest food did not affect the survival of beetles but it caused complete inhibition of egg laying by beetle females after several days of rearing. So, it can be stated that saponins are the agents suppressing the fecundity of Colorado potato beetles, however, the mechanism of this phenomenon requires more detailed studies.