-We present data on the reproduction of the endangered giant lizard (Gallotia simonyi machadoi). The general sequence of the reproductive cycle and time intervals of the different phases are described, together with courting and mating behavior patterns. Behavior patterns prior to mating included male head bobs with an inflated throat In two courtship sequences, males performed seven consecutive head bobs of almost constant duration. Sexual activity, including copulation, began during the first two weeks of May. Clutches were laid from the first week of June until the end of August Mean clutch size was 8.6 (range 5-13 eggs). Significant relationships were found between clutch size and female snout-tovent length (SVL), clutch weight (CW) and female SVL, and between CW and female body weight. Mean relative clutch mass was 0.25. Eggs began to hatch in the beginning of August; at 28-29 C the incubation time averaged 61 days. Biometric data of different morphological traits are reported for 146 newborn lizards (3-20 days old). Lizards of the genus Gallotia are endemic to the Canary Islands (Arnold, 1973). Five living species have been described; one of the largest species, G. simonyi, lives on El island. Three subspecies have been described for G. simonyi; the extinct G. s. simonyi (which lived in the Roque Chico of Salmor-El Hierro-until the end of the 1940s), G. s. gomerana, and the extant G. s. machadoi (L6pez-Jurado, 1989), which was considered extinct till 1975, when it was rediscovered (Bohme and Bings, 1975). The Giant Lizard, G. s. machadoi is presently threatened with extinction, as the only remaining population survives in a small overhanging cliff in the northeast end of the island. Population size has been estimated between 1000 (Naeslund and Bischoff, 1998) and 1600 individuals (Rodriguez-Dommnguez et al., submitted). The difficulty of climbing to the habitat and the protected status of the species has hindered study of the reproductive biology of G. s. machadoi. To date, there has been only one study made in nature (Machado, 1985). Adult animals may reach a snout-vent length (SVL) of 226 mm (Rodriguez-Dominguez et al., 1998) but indirect estimations of total body length indicated that larger animals may be present (Martinez-Rica, 1982). Gallotia s. machadoi is a diurnal, heliothermic, and omnivorous lacertid, being active almost all year round with increases in the spring-summer months. Its daily activity pattern is clearly unimodal with a pronounced peak at midday (1000-1300 European Standard Time) and lower, though roughly constant levels at later times of the day. Vegetation in its habitat is mainly composed of xerophytic succulent plants and woody shrubs (Machado, 1985). The Viceconsejeria of Medio Ambiente of The Canary Government maintains 57 adult lizards in captivity (30 males and 27 females), six of them captured from the natural habitat, at the Centro de Reproducci6n e Investigaci6n del Lagarto Gigante de El Hierro (Frontera, El Hierro); they have been the progenitors for the breeding program. The aims of our study are to present the first data on the general reproductive cycle, adult courtship and mating behaviors, and biometric analysis of the eggs and the newborn G. s. machadoi lizards. Basic reproductive data are critical in the conservation effort. For example, in order to follow the development of the newborn, these data are a necessary baseline to compare future juvenile and adult data. Moreover, as the captivity-raised young will contribute to the future recovery of the endangered wild population by releasing them in particular suitable areas, a detailed knowledge of their developmental characteristics will help to decide what animals are more suitable for re-