The deepwater redfish Sebastes mentella has a wide distribution in the North Atlantic Ocean. In the Irminger Sea, there is evidence for two phenotypes (deep-sea and oceanic) of deepwater redfish. The two phenotypes have overlapping geographic distributions but differ in depth preferences. There are two hypotheses on deepwater redfish stock structure in the Irminger Sea. One suggests that mature individuals of a single stock segregate according to size and age and therefore that the phenotypes represent different life stages of the same stock. The other hypothesis suggests that there are two different stocks and that these stocks segregate mainly according to depth. Additionally, it is not clear whether the fish of the deep-sea phenotype in the Irminger Sea and those on the Icelandic continental shelf represent one stock. Analysis of genetic variability at eight allozyme markers in 1,763 deepwater redfish from 26 samples collected at different depths in the Irminger Sea and the Icelandic continental slope showed a significant difference between deep-sea and oceanic samples, suggesting the presence of two distinguishable stocks. This is supported by (1) significant heterozygote deficiency at most loci in pooled samples, (2) extensive allele frequency differences between samples classified as belonging to deep-sea and oceanic phenotypes, and (3) clustering of deepwater redfish samples of the same phenotype in a neighbor-joining dendrogram and in Bayesian analyses (STRUCTURE and the ΔK procedure for determining the number of clusters). The results indicate that deepwater redfish stock structure should be taken into account for sustainable fisheries management in the Irminger Sea and adjacent waters.