The Mercury Islands tusked weta, Motuweta isolata (Orthoptera: Anostostomatidae), occurs naturally only on Middle Island, a 13 ha island in the Mercury Island group, New Zealand. Growth and development were studied to support a conservation programme to establish them on two other islands and thereby reduce chances of accidental extinction. The egg incubation period was determined from 784 eggs laid by eight weta (range 6-275 eggs per female) exposed to varying temperature similar to Middle Island. Most eggs (240) hatched after 12-39 weeks but 17 eggs laid in late winter hatched after 48-63 weeks. Overall, 87% hatched between October and December, and all hatched between August and February. Egg mortality was 67% (range 50-100% for individual females). Growth and postembryonic development was observed in the laboratory at 16-18°C, and a 14 : 8 h (light : dark) photoperiod. Males and females became adult at either the 10th or 11th instar. Lengths of the head, head with jaws, pronotum, protibia, tympanal membranes, mesotibia, metatibia, metafemur, cercus, ovipositor, tusks, hypandrium and stylus are given for each instar. Tusks appeared as slight bulges on the mandibles of males at the 7th instar, and subsequently enlarged allometrically at each ecdysis. The lifespan from hatching averaged 718 ± 51 d (±95% CI) and ranged from 595 to 902 d. No significant differences were detected between the lifespans of either males or females, or weta that became adult at the 10th or 11th instar. Adult status was confirmed for one 10th instar male when it copulated with 11th instar adults. Males made multiple unsuccessful attempts to copulate with two 10th instar females and 284 eggs were found in one 10th instar female after it died. Only one 11th instar female laid eggs (83) but none hatched in the laboratory.