Non-invasive diet studies, which are a simple but important tool to understand trophic interactions inside ecosystems, need to be as detailed as possible. Determining the precise biomass of ingested prey is a key to obtaining not only a better understanding of the amount of food really ingested but also the predator-prey interactions. It is particularly relevant in the case of rodents, because they are often a predominant prey in carnivores’ diet and can differ widely in biomass. This study demonstrates how an original and simple method for measuring rat incisors found in cat scats produces measurements which can be correlated with rat weight. This correlation, used in a field application, made it possible to: (i) calculate a more accurate biomass of rats in cat diets and thus obtain a better estimation of the proportion of rats compared to other prey in cat diets; (ii) show that cats preferentially ate smaller rats, indicating that the use of the mean weight of rodents sampled by trap-lines may induce a significant bias in the biomass calculation. Likewise, a correlation between rat lens weight and incisor measurements was found. Using this correlation, it should be possible to estimate the age of the rats eaten by cats and obtain a better understanding of the impact of predators on prey population dynamics. To cite this article: E. Bonnaud et al., C. R. Biologies 331 (2008).