The effects of lactosucrose (4G-beta-D-galactosylsucrose) on fecal flora and fecal putrefactive products were studied in 3 Himalayan and 5 Persian cats fed 175 mg of lactosucrose/each/day for 2 weeks. During lactosucrose administration, the counts of lactobacilli increased significantly (p < 0.05), whereas the counts of clostridia, including Clostridium perfringens and Enterobacteriaceace decreased significantly (p < 0.05). The levels of fusobacteria and staphylococci were decreased significantly (p < 0.01) on day 7 of lactosucrose administration, while the counts of bacteroides increased significantly (p < 0.05) and day 14 of lactosucrose administration compared to pre-administration. In frequency of occurrence, bifidobacteria increased significantly (p < 0.001) during lactosucrose administration, while Spirochaetaceae and lecithinase-negative clostridia decreased significantly (p < 0.05) on day 14 of lactosucrose administration compared to pre-administration. No detectable change occurred in the counts of other organisms throughout the experimental periods. Fecal concentrations of ammonia, indole, ethylphenol, and urinary ammonia were reduced significantly (p < 0.05) on day 14 of lactosucrose administration. The water content and weight of the feces increased slightly during lactosucrose administration, but the pH values decreased slightly. The environmental ammonia and the fecal odor also decreased remarkably (p < 0.01) during administration.