Sporocysts of Sarcocystis gigantea in cat faeces were placed in vented polystyrene tubes and superficially buried in both open and shaded sites. Their survival, as measured by their ability to excyst in vitro, was then monitored during the course of two separate experiments extending over 12-month periods. The results showed that the viability of these sporocysts declined most rapidly over the summer months and suggested that they were unlikely to remain infective for more than 1 year.