Photo-identification using natural markings has been used for pilot whale (Globicephala melas) studies. However, none of these studies investigated the reliabilityofthemarksused.Toidentifywhichmarktypesarereliableandwhichcould improve the method, fifteen mark types, and their distribution within the population, were described. The rates of gain and loss of each mark type were calculated and the variability in visibility was investigated. Although the mark types associated with the current photo-identification method, the notch and the protruding piece, appear to be permanent, they allowed us to identify only 33% of our sample. The prevalence of all but two mark types is independent of the identifiability of a photograph.Oneoftheseisalreadyusedinthecurrentphoto-identificationmethod. This independence indicates that the proportion of the population that is currently identifiable does not differ from the rest of the population in its susceptibility to factors causing marks, such as predation, and thus appears to be representative of thewholepopulation.Usingsaddlepatchesincombinationwiththecurrentphotoidentification method would double the percentage of the identifiable individuals. However, due to limitations of matching software, the current method is easier to use.