Abundant and well-preserved fish occur at intervals within the Romualdo Member of the Santana Formation (?Albian, Early Cretaceous) in the Araripe Basin, northeast Brazil. Preservation of large numbers of articulated individuals of similar size on a single bedding plane is consistent with the mass mortality of fish shoals over short time intervals (days or weeks). Preservation can be exceptional, with some individuals having traces of soft tissues preserved, including musculature, gut lining, gut contents and pigment of the eyes. Mass mortality assemblage density can be high, such that fossils may produce a single layer within millimeter-laminated concretions, constituting a fish bone-bed. The causes of the mass mortalities are difficult to determine but may have been multi-factorial, including changes in surface water oxygenation or salinity. Toxification of the water column by release of H2S from sediments or by toxic dinoflagellate or cyanobacterial blooms may also have been involved but firm evidence is lacking.