Data description The data in this manuscript describes the construction of marked strains of the filamentous fungus Neurospora crassa and Bayesian methods for estimating relative fitness in competition experiments. The data are from competition experiments of different Neurospora crassa strains where one strains has has a genetic marker that can be detected with a High resolution melting PCR assay. Data are in relative fluorescence units output by the real-time PCR machine. The different files are described in Readme.txt and the file csr-ms.R contains scripts in the R language (www.r-project.org) that allows repeating the analyses done in the manuscript. In depth descriptions of data files and the meaning of different columns are in those files.