Abstract: Lower Turonian inoceramid bivalves are described from the Sergipe Basin in northeastern Brazil, and a palaeoecological analysis of the region is attempted. The principal locality, Retiro 26, comprises a sequence of 35 m of the Cotinguiba Limestone Formation. Various unconformities occur in the sequence. The formation is inferred to have been deposited in an outer shelf environment of a subtropical shallow sea, with oscillating water depth, occasional strong bottom currents and ephemeral reductions of the oxygen level. The inoceramids show some endemism. Two inoceramid associations are recognized: the Mytiloides mytiloides and the Mytiloides hercynicus associations. Four new species of divergently ornamented inoceramids are described and referred to the new genus Rhyssomytiloides. Two new species of Sergipia and some previously known species of Mytiloides and Rhyssomytiloides are also described.