Modal analyses of carapace length frequencies and interpretation of biological data for the northern shrimp (Panda/us borealis) indicate variation in both age at sex change and growth rate for some Northwest Atlantic populations. The first female age (the species is protendrous) varied from 6 in the southernmost area to 7 and possibly 8 in more northerly areas. The observed differences could not be related to differences in environmental temperature. Overlapping of modes in the length frequency data was severe in many cases, possibly obscuring an additional mode (age group) in the Davis Strait data. The assumption that sex change occurs at a specific age and the choice of appropriate models and statistical analyses to describe growth require further study.