A strain of Serratia marcescens showing the ability to degrade caffeine and other methylxanthines was isolated from soil under coffee cultivation. Growth was observed only with xanthines methylated at the 7 position (caffeine, 1,3,7-dimethylxanthine; paraxanthine, 1,7-dimethylxanthine; theobromine, 3,7-dimethylxanthine and 7-methylxanthine). Paraxanthine and theobromine were released in liquid medium when caffeine was used as the sole source of carbon and nitrogen. When paraxanthine or theobromine were used, 3-methylxanthine, 7-methylxanthine, and xanthine were detected in the liquid medium. Serratia marcescens did not grow with theophylline (1,3-dimethylxanthine), 1-methylxanthine, and 3-methylxanthine, and poor growth was observed with xanthine. Methyluric acid formation from methylxanthines was tested in cell-free extracts by measuring dehydrogenase reduction of tetrazolium salt in native-polyacrylamide gel electrophoresis gel. Activity was observed for all methylxanthines, even those with which no bacterial growth was observed. Our results suggest that in this strain of S. marcescens caffeine is degraded to theobromine (3,7-dimethylxanthine) and/or paraxanthine (1,7-dimethylxanthine), and subsequently to 7-methylxanthine and xanthine. Methyluric acid formation could not be confirmed.