Several biases are often introduced in taxonomic studies due to the transformation or loss of biological information that occurs at different stages from the original depositional ecosystem to the finally selected study sample. We propose the creation of a Taphonomic Alteration Index for Fishes (TAIF) that allows an estimation of the extent of such transformation in the selected sample and its representativeness of the complete record. TAIF is designed as an easilyapplicable index that can be used to rapidly assess and compare the taphonomic alteration in different taxa or localities and can also be used as a quantitative variable in other analyses (e.g., morphometrical, statistical). To test TAIF, we have used a sample of Teleostei incertae sedis (N=190) from the Early Cretaceous of the Las Hoyas site (Cuenca, Spain). Although the obtained TAIF values show some variability, they are in general low, as would be expected for a Konservat-Lagerstätte. The low TAIF values obtained fit with the hypothesis that these fishes inhabited low energy and shallow inland waters, where the effect of anoxia and bacterial sealing would have preserved their remains from degradation. Similar indices could be designed for other taxa based on TAIF.