We studied the variation in the number of chaetae of 15 characters in Saduria entomon (L.) (Crustacea, Isopoda) from the Baltic (2 samples), White, and south-eastern Barents Seas. The following parameters were analysed: (i) means, (ii) inter-individual variance, (iii) intra-individual or stochastic variance measured by fluctuating asymmetry, i.e. random deviations from perfect bilateral symmetry, which is a measure of developmental instability. The samples from the Baltic Sea were characterised by a greater number of chaetae than those from the White and Barents Seas in almost all characters. Inter-individual variance in the Barents Sea sample was lower than in the other samples, possibly because of low genetic variation. Fluctuating asymmetry was the least in the sample from the Baltic Sea (Gulf of Gdańsk), indicating that environmental conditions in this location are the most favourable. In juveniles the magnitude of the fluctuating asymmetry and its contribution to the total variance was higher than in adults.