Three species of epiphytic dinoflagellates - Ostreopsis siamensis, O. lenticularis, and O. ovata have recently been found on both the east and west coasts of the upper North Island, New Zealand. The morphological differences of all three Ostreopsis spp. have been studied with both light and scanning electron microscopes. Detailed studies of the inner face of the thecal wall of Ostreopsis siamensis revealed two types of trichocyst pores: small, simple pores and large multipore structures. The multipore structures apparently have not been previously reported. Surveys conducted during the period from November 1995 to April 1997 showed that all three Ostreopsis spp. were fairly widespread in northern New Zealand, and all three species were found to occur south of 35°S latitude. Cell concentrations of Ostreopsis spp. in summer were substantially higher than in early spring. There was also a clear regional difference in distribution; cell concentrations of the dominant species, O. siamensis, were greatest in Rarawa and Tokerau on the north-east coast. Other less abundant epiphytic species recorded during the same period included Prorocentrum lima, P. compressum, and Coolia monotis. These species were generally more sporadic in distribution than Ostreopsis spp.