The pink bollworm, Pectinophora gossypiella, enters diapause as a last instar larvae in the fall in California and Arizona. Only diapausing larvae survive the host free winter period until spring emergence. At the onset of diapause, the fat body biosynthesizes a 103 kilodalton protein unique to the diapause state. The protein, that we call "pectinophorin," appears in the hemolymph as a lipoglycoprotein and may serve as a storage protein, but also shows cryoprotectant properties. Pectinophorin was isolated and purified from diapause larvae and partically sequenced. Total mRNA was isolated from diapause-induced larvae and cDNA segments were made using reverse transcriptase. The cDNA was ligated and cloned into the bacterial plasmid pUC-13 and recombinant colonies containing cloned DNA segments were visualized by blue/white color selection using IPTG and X-GAL. Polyclonal antibodies were made to pectinophorin. An immunoblot assay (ELISA) was developed based on the antibodies. Last instar larvae were obtained from cotton fields in Blythe, CA and Yuma, AZ throughout the fall of 1990.The ELISA tested positive for pectinophorin in the same proportion as larvae entering diapause as determined by waiting one month to determine which larvae did not pupate. Use of an oligonucleotide probe based on the pectinophorin gene revealed that trace amounts of mRNA coding for pectinophorin were present in all larvae stages of pink bollworm. Since a certain small percentage of natural populations are non-diapause larvae, the oligonucleotide probe provides a means to determine this percentage exactly.