Attenuated lines of Eimeria acervulina were isolated between 62 and 72 h post-infection from the Houghton (H) strain. The inoculation of small numbers of oocysts of precocious (HP) lines gave substantial protection to Light Sussex chicks kept on litter against challenge with the virulent H strain. The precocious trait of the 72 h HP line was shown to be stable because the kinetics of oocyst production remained unaltered after 9 consecutive passages through birds in which only late developing oocysts were used for passage. The precocious 62 h HP line was subjected to further selection for 5 consecutive passages. The resultant (62 A HP) line was shown to be more attenuated than the 72 h HP line but remained capable of immunizing chicks against challenge with the H strain.