White muscle disease in Western Australia appears to be a sporadic disease, fundamentally related to selenium deficiency but in the main limited to young sheep subjected to other concurrent stresses. Whether the complicating stress condition acts primarily at a nutritional level or whether if affects associated endocrine or metabolic pathway is uncertain from present evidence, although it is possible that both mechanisms may be involved. The disease is restricted to the southwestern agricultural districts, having rainfalls of greater than 20 inches. A much greater than average rainfall during the early winter growth stages of subterranean clover, followed a dry August and September, was common to both years (1960 and 1961) during which WMD was first widely recognized and may be ecologically significant in winter outbreaks. There are two peaks of incidence--one in late winter and early spring, affecting young lambs running mainly on subclover pastures, and the other in the summer, affecting weaners and hoggets fed cereal stubble, hay and grain. A survey of these feeds suggests that tehy are very often low in selenium, and may frequently contain less than 0.03 p.p.m. (dry matter) which is proposed as a critical level. However, many flocks grazing pastures containing less than this amount of selenium appear healthy. Ten trials were carried out in the affected area in 1960-61 to test the effect of selenium on lamb or weaner rate of growth Only one trial produced evidence of a significant response although one other suggested that an improvement had taken place. Five trials to determine the value of selenium in improving ewe fertility and early lamb survival were also conducted. The. results of two of them suggested that some improvement could be achieved. Vitamin E also appeared useful in the fertility trials. The soil and plant selenium backgrounds, insofar as thy are known in Western Australia, are discussed in relation to the pattern of outbreaks. Certain factors in the changing agricultural practices are mentioned as probable contributors, notably an increase in sheep stocking rates and heavier application of superphosphate.