The rate of growth of the head capsule in Culex territans Walker has been ascertained in 435 larvae from the population of a permanent marsh south of the Richelieu River, Quebec. The mean and range of head capsule widths have been determined for each larval instar. The rate of growth of the head capsule with larval instar follows Dyar's law. The exponential function Y=0.1939-1.5655x was accorded with the data better than a linear function.