Biogenic calcretes associated with a regional Cretaceous to Paleogene subaerial unconformity and an intraformational composite (polygenic) surface in Upper Cretaceous intra-platform peritidal successions in central Dalmatia and eastern Istria, Croatia (Adriatic-Dinaridic Carbonate Platform), were analyzed for their delta super(13)C and delta super(18)O signatures in order to provide insight into the conditions of subaerial exposure and calcrete development. The distinctly negative delta super(13)C signatures of biogenic calcretes marking the regional subaerial unconformity differ considerably from the delta super(13)C values of the host marine limestones. This indicates carbon isotope exchange of primary marine CaCO sub(3) with CO sub(2) released by root and rhizomicrobial respiration and subsequent precipitation of pedogenic calcrete. The range of delta super(13)C (from -13.1 to -8.2 ppt Vienna PeeDee Belemnite standard, VPDB) and delta super(18)O (from -10.1 to -6.1 ppt VPDB) values of calcretes are similar to those reported from calcretes elsewhere, and the delta super(13)C values of biogenic calcretes with typical Microcodium aggregates (-13.1 to -12.3 ppt VPDB) at the Sibenik locality are very close to, or at the lower limit of, values for soil carbonates formed in isotopic equilibrium with soil CO sub(2). These values are expected for authigenic pedogenic carbonates formed under the influence of C sub(3) plant communities, without influence from heavier carbon from pre-existing carbonate and lack of input of atmospheric CO sub(2). Such low delta super(13)C values support the interpretation of Microcodium aggregates as being precipitated under a direct biological control within the soil, although the relationship between formation mechanisms and stable isotope signatures of Microcodium needs further investigation. The delta super(13)C values (-4.4 to -3.6 ppt VPDB) of rhizogenic calcretes formed inside firmground Thalassinoides burrows of the composite surface at the Sibenik locality are more negative than the delta super(13)C values of the host marine limestones, which confirms that the composite surface went through a phase of meteoric pedo(dia)genesis. However, the overall delta super(13)C values of calcretes are less negative than expected, which might reflect contamination from associated primary marine carbonate. This study represents the first detailed stable isotope investigation of calcretes from carbonate successions of the External Dinarides, and the results may be applied to discontinuities present in other shallow-water carbonate rock successions.