The aim of this work was to verify if starvation influences the appearance of the definitive upper jaw inOphryotrocha puerilis(Annelida, Polychaeta). On the basis of the data obtained from two series of experiments carried out parallelly on individuals ofO. puerilis, maintained in pair culture or in isolation, the following results were obtained. Starvation did not interfere with the replacement of the juvenile upper jaw by the definitive upper jaw in the animals kept in isolation; rather, the appearance of the definitive upper jaw showed a frequency significantly lower than that shown by the controls in those maintained in pair culture and under starvation for one month. The following interpretation is suggested. Starvation by itself does not influence the replacement of the juvenile upper jaw with the definitive; nevertheless, in the presence of some other environmental factor, such as pair culture (the Paarkultur of Hartmann & Huth), starvation does exert some inhibiting influence on the replacement of the upper jaw.