The relationships between fat depots and body fat content and between protein content and protein indices were examined in American marten Martes americana carcasses obtained from trappers in northeastern Ontario during the 1995/96 fur harvest season. Percent fat and percent protein did not differ significantly between sexes (t = 0.45 and 0.55, P = 0.66 and 0.58, respectively). Protein contents did not vary much among 42 individuals (range: 14–19%), and therefore we found no significant index of percent protein (0.01 < r < 0.23, P > 0.05, N = 42). In the development phase, percent fat (PFAT) was better predicted by six of the eight potential fat indices, and better predicted in females than in males (N = 17 and 18, respectively), but the omentum dry mass (ODM) performed best with both sexes (r² = 0.69 and 0.80 males and females, respectively). During the test phase (N = 18 males, 22 females), estimated PFAT based on ODM (per sex) did not differ from observed percent fat in either males or in females (paired t = 0.01 and 1.28; P = 0.99 and 0.22, respectively). The models slightly overestimated the number of males and females of below average condition, which indicates that the models were conservative. The accuracy of the omentum dry mass fat depot appears adequate to detect changes of physical condition in harvested marten populations. The lack of variation in protein contents 1) indicates that most animals were near average protein level, possibly due to suitable habitat conditions and 2) prevented us from finding a protein index. We encourage the development of models where conditions (e.g. temperature regime, level of disturbance, carrying capacity) may be harsher for marten.