Earlier investigations into the biology of Sarcocystis are briefly reviewed; information reported since 1972 is reviewed in detail. The relative efficiency of haemagglutination (HAT, macro and micro systems), complement fixation (CFT, macro and micro systems) and the indirect fluorescent antibody test (IFAT) was studied using macrocysts (S. gigantia) from sheep oesophagi as antigen. In the HAT, macro system titres were always higher than micro system titres. Hyper immunised rabbits had higher titres than hyperimmunised sheep. Fifteen of 24 naturally infected sheep had negative titres. The macro CFT gave comparable results: the micro CFT was affected by persistent anticomplementary factors in sheep serum. The IFAT was both sensitive and repeatable. In all test systems, Sarcocystis antibody titres were minimal in infected adult sheep and in pasture-raised lambs. The value of serology in surveys of prevalence and in diagnosis of sarcocystosis is discussed. Two types of macrocyst were found in skeletal muscle of sheep at slaughter: 'fat' cysts resembled oesophageal cysts (S. gigantia) grossly and in ultrastructure of the wall; 'thin' cysts (S. medusiformis n. sp.) were narrower and ultrastructurally distinct. The relative prevalences of the three sheep macrocysts were independent. Fat and thin macrocysts were transmitted to cats and similarly sized sporocysts produced. S. gigantia sporocysts failed to infect lambs; reasons for this are discussed. Survival of S. gigantia macrocysts was studied using an oxygen electrode and by cat feeding. Macrocysts were viable after 10 minutes at 52.5°C but not after 20 minutes at 55°C or 10 minutes at 60°C. Macrocysts survived 60 days at -14°C, cysts stored at 10°C for 13 days and 4°C for 30 days metabolised vigorously. Sheep meat should be exposed to 60°C for at least 20 minutes to render it non-infective for cats. Using muscle digestion and histology, Sarcocystis spp. were found in (%; number examined); feral goats (28;60), red deer (30;50), wild pig (10;50), norway rat (84;50), mouse (8;50) and rabbit (16;50); none in 62 opossums and 8 wallabies. A goat species was transmitted to dogs (sporocysts 13.6±0.69x9.25±0.55), a rabbit species to cats (sporocysts 12.5±0.31x9. 29±0.45) and one in rats to cats (sporocysts 10.59±0.52x7.87±0.41). Appropriate sporocysts failed to infect laboratory rats or rabbits. A survey showed that feral cats inhabit and breed in a variety of terrains in most parts of New Zealand. The commonest foods eaten were rabbit (22% total reports), opossum (18%), sheep (16.6%) and birds (14.5%). The development and pathogenesis of a dog-derived species was studied in goats. Doses of 5 x 106 sporocysts caused death at 18 and 19 days after infection; necropsy revealed extensive petechial haemorrhages. Schizonts occupied endothelial cells, especially in renal glomeruli. 6 x 105 sporocysts caused death at 24 and 34 days; lesser doses caused pyrexia, anaemia, anorexia and stunting. Sarcocysts were found in muscle fibres at 34 days, appeared mature at 80 days and were infective for dogs at 129 days. Changes in levels of Hb, PCV, TP, SGOT and Saracocystis antibodies were shown. Four sheep given sporocysts were not infected. The potential importance of sarcocystosis in animal production and the need for further research is discussed.