Biserrula is an annual pasture legume endemic to the Mediterranean basin and has been recently domesticated for use in Mediterranean environments in southern Australia. Over the past 10 years the species has been associated with isolated cases of what appears to be a primary photosensitisation in sheep that graze green pastures in winter and spring. Whole-top samples of biserrula pasture were taken from a range of farmers’ paddocks over 2 years (including paddocks where photosensitisation had been observed) and methanolic extracts were screened by high-performance liquid chromatography and liquid chromatography coupled to mass spectrometry for known primary photosensitising compounds belonging to the classes furanocoumarins and dianthrones. None of these were detected. Pyrrolizidine alkaloids were also not detected and this supports the view that a secondary photosensitisation is not involved. Chlorophyll profiles were relatively unchanged between samples and this suggested that chlorophyll metabolites are unlikely to be responsible for a primary photosensitisation. A series of luteolin and apigenin mono- and diglycosides were identified in the extracts, but these are not regarded as photosensitising compounds. Further work is required to establish the photosensitising agent(s) in biserrula.