The study was conducted to investigate the effect of ammonium bicarbonate and casein on rumen fermentation and relative populations of fibre-associated cellulolytic microbes in untreated and sodium hydroxide treated rice straw in vitro. Form of nitrogen had different effect on rumen fermentation and fibre-associated microbial populations. Cumulative gas production significantly increased with addition of nitrogen. Addition of casein significantly improved rumen fermentation and microbial populations. Casein had higher efficiency to supply available nitrogen to microbial growth and degradation of straw than ammonium bicarbonate. It is inferred that a supply of protein nitrogen would be needed for maximum degradation of straw.