Symbiotic bacteria use numerous strategies to compete for their host niches, but how intraspecific competition is regulated by environmental cues during host colonization was unknown. Speare et al. found that liquid viscosity regulates type VI secretion system (T6SS)-mediated killing in Vibrio fischeri during habitat transition. Using a liquid hydrogel medium that mimics the high viscosity of the mucus-rich host light organ of the Hawaiian bobtail squid Euprymna scolopes, the authors found that exposure to high viscosity increases T6SS expression and sheath formation in vitro and activates T6SS-mediated killing after only 30 minutes. High viscosity was also found to promote co-aggregation of different genotypes that naturally compete for the same niche, thereby facilitating the cell–cell contacts that are required for interbacterial competition.