In dry-extruded canine diets, starch ileal digestibility varies with the starch source, amount and processing parameters. Starch that escapes duodeno-ileal digestion can affect faecal quality by stimulating colonic bacterial fermentation. The aim of the present study was to assess the effect of various resistant starch (RS) sources and levels on the faecal score of dogs of different breeds and sizes. A total of twenty-one healthy adult female dogs (body weight ranging 5·0–30·6 kg) were used. The maintenance diet for the dogs was supplemented with increasing amounts of RS from two sources: high-amylose starch from maize (to 2·5, 4·3 and 7·4 % RS) and raw potato starch (to 7·4 and 11·4 % RS). Each level of RS was tested over a 7 d period followed by a 7 d washout period. Faecal scores were evaluated by one person using a scale ranging from 1 (for hard and dry faeces) to 5 (for liquid stools). Faeces were considered ‘optimal’ at scores of 2·5–3·0, ‘acceptable’ at scores of 3·0–3·75 and ‘unacceptable’ at scores >3·75. Small dogs showed very little sensitivity to RS based on the faecal score, while large dogs were quite responsive to RS supplementation. These results suggest that small dogs are poor models for assessing the effect of starch sources on ileal digestibility. They also indicate that a low RS content (strongly affected by source and processing) is an important factor for ensuring an optimal faecal score in large breed dogs.