The clinical course of an outbreak of keratoconjunctivitis in housed lambs and their dams was followed. Signs were transient generally and became severe in only a small proportion of lambs. The outbreak became most obvious when the lambs were 46 to 55 days old, when 46.9 per cent were affected. Mycoplasma conjunctivae isolations, confirmed by comparison with the type strain by biochemical and serological reactions, increased to 62.1 per cent of all eyes swabbed, but no correlation could be demonstrated between presence of the organism and clinical status. The reasons for this are discussed. Mycoplasma ovipneumoniae was also recovered from the eyes of a small number of lambs. Instillation of a broth culture of M conjunctivae into the conjunctival sacs of four hoggs produced a transient keratoconjunctivitis similar to that observed in the field, but no effect was observed in animals inoculated intravenously. M conjunctivae may therefore be the aetiological agent of non-follicular infectious ovine keratoconjunctivitis, although further work in gnotobiotic or specific pathogen free lambs is required to establish the fact beyond doubt.