We examined lateral plate variation in populations of stickleback, Gasterosteus aculeatus, from the rivers Miño and Limia in Galicia (Northwest Spain), by analysing morphs, number of lateral plates, and position of the plates. In order to test for sexual dimorphism, the means and variances of plates were compared in males and females of some populations. No significant differences were found between sexes, therefore all remaining populations were analysed without controlling for sex. As stickleback populations in Galicia have an annual life span, the populations from the river Miño were divided in two age groups to compare lateral plate number. Analysis of each group revealed significant differences among populations, in accordance with the large amount of variation found in populations from other regions. Analysis of samples collected from the same place and at the same time of year, but in different years, indicated that there was little year to year variation. This suggests that populations are well adapted to their habitat, or that environmental conditions did not change significantly during the period of sampling. Finally, we propose a new method for the evaluation of scute variation analysis of the position of lateral plates. Cluster analysis of our data clearly differentiated populations from the rivers Miño and Limia.