Highlights : Members of the order Legionellales are best known as major intracellular pathogens, including the agent of Legionnaires’ disease and the agent of Q fever. Most of our knowledge is currently biased toward the pathogenic species of the Legionellales.Recent ecological surveys have uncovered an unexpected diversity of Legionellales associated with nonvertebrate hosts. Some of these novel Legionellales use alternative strategies to persist in nonvertebrates, including a complex mutualistic lifestyle.This diversity makes Legionellales particularly interesting for studying the mechanisms underlying bacterial lifestyle transition, including the emergence of pathogenicity and mutualism, host specificity, and transmission. Investigations on nonpathogenic Legionellales are now needed to identify specific molecular mechanisms driving adaptations to different hosts and lifestyles.