An understanding of the nitrogen compounds required for growth of ruminal bacteria is of importance to optimising ruminal fermentation and in formulating optimal protein requirements of ruminants. Bryant (1973) concluded that cellulolytic ruminal bacteria used only NH3 as N source for growth. The main species were unable to grow on other nitrogen sources in the absence of NH3, and the incorporation of radioactive pre-formed amino acids appeared to be minimal. Disappearance of NH3-N from the growth medium was equal to N incorporation into bacterial protein. As a consequence, the assumption was made in drawing up the Cornell Net Carbohydrate and Protein System (CNCPS) that structural carbohydrate-fermenting bacteria use ammonia as their only source of nitrogen (Russell et al., 1992). More recent studies with mixed cultures suggest, however, that fibrolytic bacteria incorporate significant quantities of amino acids (Griswold et al., 1996; Carro and Miller, 1999). The present experiments were undertaken to clarify the N sources for growth of the three main species of cellulolytic ruminal bacteria in pure culture.