The survival of sporulated Toxoplasma gondii oocysts in water at -10 C to 70 C for various periods was investigated. Infectivity of T. gondii was tested by bioassay in mice. There was no marked loss of infectivity of oocysts stored at 10 C, 15 C, 20 C, and 25 C for 200 days, whereas there was a 100-fold loss of infectivity of oocysts stored at 30 C for 107 days. Oocysts stored at 35 C were infective for 32 days but not 62 days, at 40 C oocysts were infective for 9 days but not 28 days, at 45 C, oocysts were infective for 1 day but not 2 days, at 50 C oocysts were infective for 1 hr but not 2 hr. At 55 C and 60 C oocysts were rendered noninfective in 2 and 1 min, respectively. Oocysts remained infective up to 54 mo at 4 C and there was no loss of infectivity in oocysts stored for 106 days at -5 C and at -10 C and for 13 mo at 0 C.