The harpactorine assassin bug, Zelus tetracanthus (Stål) (Hemiptera: Reduviidae), is an insect predator frequently found in surveys of saltcedar (Tamarix spp.) in Oklahoma. Previous studies in a laboratory showed it could prey on Diorhabda carinulata (Desbrochers) (Coleoptera: Chrysomelidae), a recently introduced biological control agent for Tamarix. An F2 cohort of Z. tetracanthus was reared from egg to adult to measure stage-specific development rates and describe immature stages at 22 ± 1.0°C and 16:8 light:dark hours. The mean times for development of the five instars were 6.96 ± 0.13, 5.18 ± 0.13, 6.94 ± 0.29, 10.92 ± 0.43, and 17.93 ± 0.76 days, respectively. Several anatomical characters to distinguish instars, including body size, body color, and select morphological characters were described. Data from museum collections of Z. tetracanthus suggested the insect has a univoltine life cycle. The development data contrast with a previous description of Z. socius (= Z. tetracanthus) and illustrate the need to further study the variable morphology, natural history, and genetic makeup of the species.