The almost complete translation of shell accretion to vertical growth was a successful morphogenetic adaptation of rudist bivalves, which replaced coral-algal-hydrozoan communities in most Tethyan shallow-marine environments during the Cretaceous. Growth rates of rudist shells are, therefore, important in reconstructions of Cretaceous shallow-marine benthic communities. Carbon and oxygen isotopic compositions were analyzed in sclerochronological profiles through the shells of two Late Cretaceous rudist bivalves from Greece and Turkey. Values of δ13C vary between +0.8‰ and +2.2‰ in the hippuritid Vaccinites ultimus and between +0.8‰ and +2.6‰ in the radiolitid Gorjanovicia cf. costata. Ranges in δ18O are −5.3‰ to −2.9‰ and −4.6‰ to −2.1‰, respectively. A distinct cyclicity of δ13C and δ18O in both shells reflects Late Cretaceous seasonality and indicates annual vertical-growth increments of 45 mm in V. ultimus and 54 mm in Gorjanovicia. The concentrations of minor elements are similar to those of extant bivalves with low-Mg calcite shells.