Boyeria irene is mainly a West Mediterranean species. The instar distribution during winter is that of a `summer species¿ sensu Corbet (1964). Little is known about how the geographical location of the population may affect the biometric peculiarities of B. irene larvae. Eight biometric variables were studied in male and female larvae belonging to three southern Iberian populations, with the objective of ascertaining whether there are differences between populations. The southernmost population (Los Alcornocales) shows the largest sizes for most of the variables measured, while the northernmost population (Sierra Madrona) shows the smallest sizes for most of the variables. Winter water temperatures may be the cause of the size divergences, due to a longer arrest in time of larval growth in the northernmost population.