During the Middle Triassic time two major transgressions (Anisisn and Ladianian in age) took place over the Tethys domain affecting the western border of the Iberian Plate. Classically three characteristic lithological units have been recognized in the Middle Triassic, traditionally named in Spain, Lower, Middle and Upper Muschelkalk. In order for comparison the same designations are kept in the present work. Lower Muschelkalk comprises the Albarracian Formation, and Upper Muschelkalk – the Tramacastilla and Royuela Formations. In this paper the attention is drawn to the main facies of these three formations and especially to the foraminifers found for the first time in their sediments. As a result of the analysis of the stratigraphical significance of the foraminiferal assemblages we refer the Albarracian Formation to the Pelsonian substage of the Anisian stage, and the Tramacastilla Formation to the Longobardian substage of the Ladianian stage. The deposits of the Royuela Formation contain Upper Ladinian - ?Carnian foraminiferal associations.