Abstract.Seasonal variations in mosquito larval head capsule width and adult female wing length were investigated in a field population ofAnopheles merusDonitz at Nceswana Lake, Ophansi, within the endemic malaria area of Natal, South Africa. An inverse relationship was detected between each of these morphological characters and seasonal fluctuations in air/water temperatures. Mean head capsule width in all larval instars decreased by 4.8‐7.9% in summer, while mean wing length decreased by 19.6%. These changes are discussed in relation to the annual range in mean air temperature in southern Africa and the distribution ofAn. merus.Implications for the use of such morphological characteristics in existing taxonomic keys are discusse