<jats:title>Summary</jats:title><jats:p>Topsoil herbage and faces collected during an outbreak of ryegrass staggers in sheep were examined for tremorgenic peniecllia. No such fungi were recovered from the plant material, but they were found among the predominant fungi in the soil and faecal samples. The commonest species of <jats:italic>Penicillium</jats:italic> and almost the only tremorgenic species encountered, was <jats:italic>Penicillium Janthinellum</jats:italic> Biourge. When fed to sheep, the mycelium of this fungus evoked a number of the clinical signs seen in field eases of ryegrass staggers. Two tremorgenic toxins were isolated from the mycelial felts and available evidence indicates that they are verruculogen and funitremorgin A. P. janthinethum also produced these tremorgens when from this study are in accord with the hypothesis that ryegrass staggers is caused by tremorgenic mycotoxins.