The physical form of the diet fed to laboratory animals should be evaluated to reduce experimental variations and confounding factors. This 14-d study evaluated the effects of diet form (pelleted or extruded) on intracage ammonia concentrations, feed disappearance, body weight, cage weight, and the degree of cage soilage and whether these effects were influenced by strain or stock or sex. Mice (C57BL/6, ICR, and nude; age, 4 wk) were randomly assigned to 4 treatment groups representing pelleted and extruded diets from each of 2 vendors (pelleted diet groups, P1 and P2; extruded diet groups, E1 and E2). Intracage ammonia concentrations depended on strain or stock, diet, and day and were higher in cages housing nude mice that consumed P1. Diet type did not affect the weight of mice at the end of the study. Feed disappearance was dependent on diet type and mouse strain or stock and was greatest in the cages of mice that consumed P1. In addition, the greatest feed disappearance was seen with ICR mice, whereas the least was seen with C57BL/6 mice. Cages housing male nude mice had greater cage soilage than those housing female nude mice. The degree of cage soilage was influenced by diet type and day also. These results show that diet form and mouse strain or stock significantly affect intracage ammonia concentrations, feed disappearance, cage weight, and the degree of cage soilage.