AbstractProchlorococcus is the smallest oxygenic phototroph in the ocean, where it can be found in great abundance throughout the euphotic zone in mid‐latitude waters. Populations of this picocyanobacterium have been observed below the euphotic zone, but the viability of these cells is unclear. To explore the tolerance of Prochlorococcus to extended light‐deprivation, we subjected multiple strains of Prochlorococcus to varying periods of darkness and examined their ability to recover when placed back in the light. Some strains recovered after 35 h of darkness while others could not; this variability was not related to whether the strains were members of high‐ or low‐ light adapted ecotypes. The presence of a marine heterotroph, Alteromonas macleodii MIT1002, in the cultures extended their ability to survive prolonged darkness, in the most extreme case by 11 d. This could be attributed at least in part to the reduction of hydrogen peroxide in co‐cultures, consistent with known roles of “helper bacteria” in detoxifying hydrogen peroxide, and this effect could be mimicked to some degree by the addition of a known hydrogen peroxide quencher, sodium pyruvate. The addition of glucose alone to the cultures provided marginal enhancement, but when both pyruvate and glucose were added together, all strains were able to survive longer in darkness than they were with only the heterotroph added. Thus, it appears that Prochlorococcus dark‐survival depends on a multitude of factors. Limited analyses of Synechococcus suggest that its dark‐survival capacity is longer than that of Prochlorococcus, for reasons that are not yet clear.