The influence of season on growth of the sea urchinEvechinus chloroticus(Valenciennes) was studied in 2 populations at Goat Island Bay (36°16′S, 174°48′E) on the north‐east coast of the North Island, New Zealand. The growth of individually tagged sea urchins in the populations was recorded at monthly intervals to determine seasonal growth patterns. Ages were determined by counting annually formed rings in their genital plates. The effect of tagging on annual growth rates of sea urchins was determined by comparing the sizes of tagged and untagged individuals of the same ages at the end of the study. Growth of tagged individuals was seasonal and their annual growth rates decreased with increasing size. Monthly growth rates of the tagged sea urchins were lowest in winter, highest in early spring, and declined sharply during late spring and early summer. Tagging did not significantly affect the growth of tagged compared to untagged individuals during the study. Regression of the sizes of tagged sea urchins after 1 year on their initial size suggested that the growth of this species does not conform well to commonly used growth models such as the von Bertalanffy equation.