Readme- Zhou et al Group size and optimal response to QS All data files have been saved in tab delimited format. Bacterial density data are provided in cfu per micro litre- all cadavers were homogenised in 500 microl of water. Relative fitness calculations- we provide initial and final proportions and final population density as well as columns labelled as "PreMalth" - these refer to the ration of final and initial population size and are calculated as: (population density of competitor (competitor A or B) X 500)/ (initial proportion (A or B) X 50). To calculate W or relative fitness of competitor A from these calculate the ratio of log base 2 (preMalthA)/ log base 2 (preMalthB). Bgal and lecithinase expression data are the peak values from times series and 5 and four experiments respectively. For lecithinase this was 1 hour after the start of experiments for Bgal this was 2 hours after the start of experiments