Over 100 Mediterranean monk seals, members of a population inhabiting the Cap Blanc peninusula in north-west Africa, died mysteriously in a two-month period in 1997. The mass mortality of this endangered species has been attributed to a morbillivirus. But an alternative & or additional & explanation is that the seals died because they had eaten fish contaminated by phycotoxins produced by a bloom of algae. The controversy demonstrates the practical and conceptual problems in investigating unusual cases of mortalities in wildlife.