Sejus togatus CL Koch, 1836 was studied using video recording of living individuals reared on nematodes in the laboratory at 20–21°C. We report the duration of juvenile stages and intra-stage changes in body length and width in juveniles and male and female adults. We describe the moulting behaviour and three-dimensional arrangement of selected setae in living larvae and protonymphs. In both males and females, the larva was the shortest (♀: 5.7 days, ♂: 6.6 days), and duration of the deutonymph was the longest (♀: 13.3 days, ♂: 11.6 days). Most juveniles increased their size significantly from the first day of life to the last day before moulting. There was no change in size from the final size of one instar to the initial size of the next instar. During the first 15 days after the last moult, males increased significantly in width, and females in length. Eventually, females were significantly longer and slightly wider than males.