Temperature-sensitive mutations, at the prune locus (pnts) of Drosophilia melanogaster, were induced with EMS. The majority of the new class of mutations, designated pnts-e, do not interact with the mutation Killer of prune (Kpn) at all of the experimental temperatures, i.e., pnts-e, Kpn flies do not die. One mutation, designated pnts-ek, is temperature-senstive with respect to both the eye coloration and the interaction with Kpn. pntr (temperature-non-sensitive), pnts-e, and pnts-ek alleles were found to differ with respect to both quantity of drosopterines and the color of the eyes. The temperature-sensitive period (TSP) for pn eye color occurs at a late pupal stage. The TSP for the pn--Kpn interaction lasts from the beginning of the first laraval instar to eclosion [22]. It is concluded that all pn mutations known to date occupy the same functional unit. The various pn mutations affect the same metabolic step to various degrees: the eye color phenotype results from a reduction in the end product, while the interaction with Kpn depends on the amount of the accumulated precursor. No recombination between pn alleles was found among more than 106 zygotes that were scored. The reasons for this failure are discussed.