Amidated pectins were tested in cultures of the pig colon content. The cultures were incubated under CO₂ at 39 °C for 24 h and then production of volatile fatty acids (VFA), gas and ammonia were measured. Citrus pectin used for syntheses of amidated derivatives served as a control. Production of VFA and gas were significantly lower in cultures with amidated pectins than in control cultures. Production of VFA and gas correlated negatively and that of ammonia positively with the degree of amidation. The length of the carbon chain of alkyls had no effect on fermentability of modified pectins. Amidation of pectin reduced the number of colon bacteria capable of growing on the pectins by one order of magnitude. Gramnegative rods were the most numerous pectinolytic isolates.