An analytical procedure has been developed for the in situ measurement of calcium isotope composition of carbonates with a spatial resolution of 15-20 μm on a Caméca IMS 1270 ion microprobe. By using two Faraday cup detectors, the 40 Ca and 44 Ca can be measured simultaneously, improving the internal reproducibility. Instrumental mass fractionation (IMF) of calcium isotopes was observed to be independent of primary ion beam intensity and of the Mg content of the carbonate, but can depend on vacuum conditions. Three calcite reference materials were used in this study (ENS 0, MEX and BRET 105E) and their δ 44 Ca values relative to NIST915a were reproducible within a typical 1σ standard deviation of ≈0.15‰. This analytical procedure was applied to planktonic foraminifera, Globorotalia inflata, dated at 2.8 Ma from Shatsky Rise (ODP leg 198). The range of measured δ 44 Ca within a single test is 1.7‰. This intratest variation can be attributed to several processes such as temperature variation, ontogenic effects or differences between primary and secondary calcite (i.e. calcite precipitated by different biomineralization processes). Despite this intratest variation, the averages δ 44 Ca for each foraminifer are similar and are in agreement with published δ 44 Ca values measured for this age. This study shows that in situ δ 44 Ca measurements in tests of foraminifera are an appropriate tool for investigating biomineralization processes.