Read me file for ÒThe rate and effects of spontaneous mutation on fitness traits in the social amoeba, Dictyostelium discoideumÓ by DW Hall, S Fox, JE Strassman and DC Queller. There are eight files associated with this paper, one for each fitness component that was measured. Each file is a tab delimited text file. Details on how each was measured are given in the paper. For seven of the files, the format of each data file is essentially identical. These files are: Hall_et_al_Plate_Growth.txt, Hall_et_al_Liquid_Growth.txt, Hall_et_al_Slug_Distance.txt, Hall_et_al_Total_FB.txt, Hall_et_al_Spores_per_FB.txt, Hall_et_al_Total_Spores.txt, and Hall_et_al_Spore_germination.txt. Columns 1and 2 and sometimes 3 give the raw fitness data for each replicate of each MA and control line, and also the ancestor replicates. Note that most MA lines were scored only once and so there are few replicates. The next column (Òmean of replicatesÓ) includes the means of the replicates for each MA and control line and has all of the ancestor replicates. The next column (Òmean of ancestor replicatesÓ) has a single entry, which is the mean of the ancestor replicates. To obtain relative fitness we divide each entry in the Òmean of replicatesÓ column by the mean of the ancestor replicates. The last column gives the mean and variance of the ancestors, the controls and the MA lines. The last file, Hall_et_al_Competitive_fitness.txt, has the data for competitive ability against the ancestor and, as such, is already on a relative fitness scale. In that file, the first 333 lines are the data for MA lines versus the ancestor, the next 185 lines are for the ancestor competing against itself (to get insight into error variance), and the last 63 lines of the file are for the control lines versus the ancestor. For each MA or control line versus the ancestor, there is a paired replicate: in the first, the ancestor is labeled and the MA line is unlabeled, and in the second, the ancestor is unlabeled and the MA line is labeled. Labeling allows us to distinguish MA/control line cells and spores from ancestor cells and spores. Labeling was used to estimate the proportion of labeled cells and spores, which are then converted to the proportion of cells and spores from the MA line or control versus the ancestor, and then used them to calculate a relative fitness measure (Equation (1) in manuscript).