The development of a naturally acquired coccidia infection in lambs in Otago is described using data derived from faecal oocyst counts and specific differentiation of the oocysts. Oocysts first appeared in faeces of lambs 19–37 days after birth. Oocyst production reached a mean peak of 250 000 oocysts per gram when lambs were 40–50 days old and slowly declined with increasing age of lambs. Coccidial infections did not appear to have any obvious effect on weight gain. The numerically most numerous species of coccidia wereEimeria ovina, E. crandallis, E. ninakohlyakimovae, E. parvaandE. weybridgensis.