The viability of Notocotylus attenuatus metacercariae was 80% at 20 wk post-encystment (PE) and decreased to 10% at 24 wk PE. Cyst viability was influenced by the duration of cercarial swimming activity prior to encystment, by the occurrence of cyst associations, and by the type of cyst storage. This is the first report on cyst associations formed by Notocotylidae cercariae. Cyst associations were formed only by cercariae encysting shortly after their emergence from snails. In cyst associations metacercariae did not overlap, but were separated by regular distances ranged from 20.4 to 24.5 μm x̄ = 22.3 μm, SE = 0.41. The mucoid materials which formed the external cyst wall covered the areas between parasites seven if distances were comparable with the cyst size. In cyst associations, metacercariae in the water and those located close to the water were viable up to 24 wk PE. Cercariae with extended swimming activity did not form associations even when present in numbers, ≤50 cercariae per cm³. The cysts established by these cercariae had a thin external cyst wall, and were not viable by 12 wk PE.