The rate of growth of the head capsule widths in Culex quinquefasciatus Say has been ascertained in larvae reared individually in the laboratory. The mean and range of the head capsule widths have been determined for each larval instar. There is no overlap between successive instars, making it possible to distinguish among them. A sexual dimorphism appears at the 3rd instar, head capsules being significantly larger in females. The rate of growth of the head capsule with larval instar follows closely Dyar's rule. The exponential function Y=0.200 . 1.563x accords with the data better than a linear function.