Ann Bucklin, Alyssa M. Bentley, and Sara P. FranzenThe sibling species, Pseudocalanus moultoni and P. newmani, occur sympatrically in the waters over Georges Bank, and may reproduce throughout much of the year. DNA sequence variation of two mitochondrial genes, 16S rRNA and cytochrome oxidase I (COI), provided conclusive evidence of the presence of two, genetically divergent taxa of Pseudocalanus; sequence differences were similar to those between species of calanoid copepods. Based on the COI sequence data, species-specific oligonucleotide primers were designed to allow rapid, molecularly-based discrimination of the species. Species-specific PCR amplification was used to discriminate adult females of the two species and to map their distributions and relative abundances over Georges Bank during April, 1996. The two species showed significantly different distributions, based on samples of 15 to 30 female individuals from 12 collections across Georges Bank. Although it is not possible to characterize the species' distributions based on one snapshot, the results of this study suggest that P. moultoni is more abundant on the northern portion of Georges Bank, while P. newmani is more typical of offshore waters. All four samples in which P. newmani predominated were collected outside the 60 m isobath of Georges Bank; one of these samples was also the only sample collected from water intermixed with Slope Water. Ongoing studies will determine the stage-specific distributions of the two species in time and space (based on samples from monthly U.S.GLOBEC Broadscale Surveys during 1996 and 1997), and will help understand whether either or both species exhibit life history patterns that aid retention on Georges Bank.