The rudist bivalves were one of the many and varied groups of organisms to be extinguished at the end of the Cretaceous Period. They were a group of bivalves which evolved during Late Jurassic times to dominate the carbonate shelves on the margins of the Tethys Ocean during the Cretaceous Period. Through Late Cretaceous times their diversity climbed to a peak and then entered a period of rapid decline, resulting eventually in the complete extinction of the group. Theories as to the cause of that extinction should clearly be based upon a knowledge of the detailed pattern of the decline. Most important is the question of the timing of the extinction: How long did it take from the peak of diversity to the elimination of the entire group? Is there one main extinction event - or are there several - or is the pattern a gradual decline? In answering these questions this work adopts a new approach to dating end Cretaceous strata by using strontium isotope stratigraphy. The method works by measuring the 87 Sr/86Sr of palaeo-seawater preserved in marine carbonate, such as the thick low-Mg calcite layer of rudist shells. The 87Sr/86Sr of seawater was changing fairly rapidly through time in the latest Cretaceous. The pattern of change has been established in detail using samples from Boreal sequences of which the ages are known with respect to the belemnite stratigraphy. Using this as a standard graph, Tethyan rudist samples have been dated by a comparison of the Sr isotope ratio. The use of Sr isotope stratigraphy has enabled a time axis to be established, against which the ranges of rudist bivalves and of their facies have been ploued. From these data it can be clearly seen that the ranges of many of the established biostratigraphic markers are in error and that the stage boundaries, as defined by the belemnite and planktonic foraminiferal stratigraphies, are offseL When the pattern of rudist diversity is plotted against this time scale it can be seen that, at a specific level, the rudists were at their most diverse at the Campanian/Maastrichtian boundary. Their decline lasted until I almost the end of the Maastrichtian after which the only survivors are Tertiary forms. The decline is related to the disappearance of rudist facies with the end Cretaceous regression, though that graph is slightly displaced from that of rudist diversity. This shows that the rudist extinction is not merely due to non-exposure of terminal Maastrichtian strata.