A study was made of the effects of temperatures and chemicals on the unsporulated and sporulated oocysts of Eimeria stiedae. In general, oocysts were affected by temperatures lower than -10 C and above +39 C, while normal development occurred between these temperatures. Unsporulated oocysts were more tolerant to temperature extremes, as they survived up to 8 days at 39 C and 12 days at -28 C. Sporulated oocysts were killed after 24 hours storage at 39 C and 4 days at -28 C. Oocyst resistance to low temperatures was not improved if unsporulated and sporulated oocysts were conditioned before and after storage at a low temperature. Presumably, the conditioning period only increased the time oocysts were exposed to cold temperatures and, therefore, reduced their development. Most chemicals and bactericidal agents had little effect on the oocysts. Only ethyl alcohol (95%), ammonium hydroxide (1 M), phenol (.1 M), and mercuric chloride (10-5 M) of those tested prevented sporulation of unsporulated oocysts. With the exception of ammonium hydroxide (1 M), sporulated oocysts developed after treatment with these chemicals.