After its formation as a single sedimentary entity the Great Artesian Basin provided a marine environment only during the Cretaceous Aptian, Albian and early Cenomanian in South Australia. Santos' Oodnadatta 1 well (1957), continuously cored through the early Cenomanian-Aptian section, gave valuable control of the sequence of and relationships between foraminiferal and molluscan faunas. Material studied for this report was obtained from 255 surface samples, collected mostly between 1957 and 1963, and from retained drill cores and cuttings of 16 petroleum exploration wells and water bores dating back as far as 1883. The foraminifera consist of 134 species, 66 of which are described as new, representing 57 genera, two of which are new; the molluscs consist of 88 species, 18 described as new, belonging to 43 genera of which two are new. The assemblages show little variation over wide areas and indicate deposition under sluggish or stagnant conditions ranging from brackish swamp to offshore facies. Four foraminiferal zones are recognizable. The faunas cross formation boundaries near the base of the marine sequence, giving evidence of marine transgression in a westerly direction during the Aptian.