SYNOPSIS. Fermentation of wheat by a clonal population of Ophryoscolex purkynei obtained from an in vitro culture yielded butyrate, acetate, smaller amounts of propionate, and traces of formate and lactate. H2 and COz were formed in a ratio of approximately 2:1. The soluble sugars tested were not fermented in manometric experiments. Decomposition of pectin by O. Purkynei is due to pectin esterase and transeliminase; the latter enzyme resembles the bacterial transeliminase rather than the fungal pectin transeliminase. The occurrence of these enzymes seems odd since the end products do not appear to be metabolized.The rate of fermentation of wheat by washed suspensions of protozoa agreed with the rate determined by following gas production in a normal culture during a 24‐hr period. The rate was lower than that reported for the holotrichous rumen ciliates; the total products formed per cell per hour by the washed suspension were 0.295 mpM.The formation of gas by fermentation of soybean and linseed oil meals was due to the starch and not protein in these materials. Manometric experiments with the pure proteins, casein, gliadin, and glutenin showed no gas evolution. Ammonification of these substrates in addition to plant chloroplasts was determined. Linseed oil meal gave a high value of 0.02 mμM/cell/hr which is of practical magnitude when compared to the rate of formation of acid, 0.08 mμM/cell/hr.Based on a concentration of 4000 protozoa/ml, estimation of the proteolytic activities of O. Purkynei indicated that it can account for 1.5% of the total ammonification. But, it can account for nearly 11% per day of the nitrogen supplied to the host when the protozoa are passed on to the abomasum. Its contribution to the volatile acids produced in the rumen is 3%, assuming the maximum rate of acid formation from wheat. Ophryoscolex can be of significance in the rumen if it occurs at high concentrations (4W/ml) but the rate of fermentatiodcell is too low to assign much importance to this organism when it occurs in low numbers.