Freshly isolated oocysts were sporulated at 29°C with constant aeration. Minimum sporulation time was determined by dosing the developing oocysts at intervals of 1 h to successive groups of chickens to assess the first occurrence of infective forms. Microscopic analysis of the sporulating oocysts allowed calculation of the time required for 50% of the oocysts to complete the process. This was considered to be the most accurate determination and the most useful for comparative purposes. For 7 species of coccidia in the fowl, the 50% sporulation times were as follows: Eimeria acervulina 11·4 h; E. brunetti 38·3 h; E. maxima 38·1 h; E. mivati 19·0 h; E. necatrix 19·7 h; E. praecox 24·8 h; E. tenella 21·2 h.