Seven field trips to Curtin Springs Station, in the south of the Northern Territory (two in July and five in December-January), were carried out between July 1984 and January 1991 to investigate the reproductive activity of spinifex hopping mice in the natural environment. Gonadal activity was determined from most samples of animals collected. Pregnant animals were present on only one occasion (December 1988-January 1989), but two females collected in December 1985 had corpora lutea in their ovaries. Most adult males were, by contrast, sexually mature, as indicated by germ-cell associations in the seminiferous tubules, spermatozoa in the excurrent ducts, and secretion in the lumina of the ventral prostates and seminal vesicles. Nevertheless, the testes were invariably very small, 2-4 germ-cell associations were sometimes present in tubule cross-sections, epididymal spermatozoa were highly pleiomorphic and seminal vesicles minute. Such morphological traits are thus invariable features of the reproductive biology of males of this species.