Caspofungin is a promising echinocandin-group antifungal agent used especially in the treatment of resistant invasive aspergillosis. The guidelines for in vitro susceptibility testing of Aspergillus species against caspofungin are not described by the Clinical and Laboratory Standards Institute (CLSI). The minimum inhibitory concentration that showed a prominent reduction of growth (MIC-2) and minimum effective concentration (MEC) endpoints are frequently used for the susceptibility testing of caspofungin as MIC determination criteria. The aim of this study was to evaluate the in vitro activity of caspofungin against Aspergillus species and to compare MIC-2 and MEC endpoints in the determination of MICs. A total of 32 Aspergillus species (18 A. fumigatus, seven A. flavus, five A. niger, and two A. versicolor) isolated from different clinical samples were included to the study. In vitro susceptibilities of the strains against 0.03-16 microg/ml caspofungin concentrations were searched by broth microdilution method as recommended by CLSI M-38A document, with the use of glucose supplemented 2% RPMI 1640 media. The MIC-2 and MEC endpoints were determined both at 24 and 48 hours. The concordance between MIC-2 and MEC endpoints of the strains at 24 and 48 hours incubations was found as 53% and 100%, respectively, with the difference of +/- 1 dilution. MIC-2 and MEC measurements showed the same values at the end of 48 hours, whereas 7% showed differences in +/- 1 dilution. MEC endpoints were also found to be more stable than MIC-2 in both of the incubation periods. In conclusion, MEC value is a more objective and stable endpoint and easier to use than MIC-2 for testing in vitro caspofungin activity against Aspergillus species.