Microbial cells secrete numerous enzymes, scavenging molecules, and signals that can promote the growth and survival of other cells around them [1Little A.E.F. Robinson C.J. Peterson S.B. Raffa K.F. Handelsman J. Rules of engagement: interspecies interactions that regulate microbial communities.Annu. Rev. Microbiol. 2008; 62: 375-401Crossref PubMed Scopus (279) Google Scholar, 2Waters C.M. Bassler B.L. Quorum sensing: cell-to-cell communication in bacteria.Annu. Rev. Cell Dev. Biol. 2005; 21: 319-346Crossref PubMed Scopus (2746) Google Scholar, 3Strassmann J.E. Gilbert O.M. Queller D.C. Kin discrimination and cooperation in microbes.Annu. Rev. Microbiol. 2011; 65: 349-367Crossref PubMed Scopus (141) Google Scholar, 4Elias S. Banin E. Multi-species biofilms: living with friendly neighbors.FEMS Microbiol. Rev. 2012; https://doi.org/10.1111/j.1574-6976.2012.00325.xCrossref PubMed Scopus (456) Google Scholar]. This observation is consistent with the evolution of cooperation within species [5Nadell C.D. Xavier J. Foster K.R. The sociobiology of biofilms.FEMS microbiology reviews. 2009; 33: 206-224Crossref PubMed Scopus (451) Google Scholar], and there is now an increasing emphasis on the importance of cooperation between different microbial species [4Elias S. Banin E. Multi-species biofilms: living with friendly neighbors.FEMS Microbiol. Rev. 2012; https://doi.org/10.1111/j.1574-6976.2012.00325.xCrossref PubMed Scopus (456) Google Scholar, 5Nadell C.D. Xavier J. Foster K.R. The sociobiology of biofilms.FEMS microbiology reviews. 2009; 33: 206-224Crossref PubMed Scopus (451) Google Scholar, 6Wintermute E.H. Silver P.A. Dynamics in the mixed microbial concourse.Genes Dev. 2010; 24: 2603-2614Crossref PubMed Scopus (144) Google Scholar]. We lack, however, a systematic test of the importance of mutually positive interactions between different species, which is vital for assessing the commonness and importance of cooperative evolution in natural communities. Here, we study the extent of mutually positive interaction among bacterial strains isolated from a common aquatic environment. Using data collected from two independent experiments evaluating community productivity across diversity gradients, we show that (1) in pairwise species combinations, the great majority of interactions are net negative and (2) there is no evidence that strong higher-order positive effects arise when more than two species are mixed together. Our data do not exclude the possibility of positive effects in one direction where one species gains at the expense of another, i.e., predator-prey-like interactions. However, these do not constitute cooperation and our analysis suggests that the typical result of adaptation to other microbial species will be competitive, rather than cooperative, phenotypes.