Many recent papers have deepened the state of knowledge about lactic acid bacteria (LAB) in fish gut. In spite of high variability in fish microbiota, LAB are sometimes abundant in the intestine, notably in freshwater fish. Several strains of Streptococcus are pathogenic to fish. S. iniae and Lactococcus garvieae are major fish pathogens, against which commercial vaccines are available. Fortunately, most LAB are harmless, and some strains have been reported for beneficent effects on fish health. A major step forward of the recent years was those converging evidences that LAB can stimulate the immune system in fish. An open question is whether viability can affect immunostimulation. The issue is crucial to commercialise live probiotics rather than inactivated preparations or extracts. There was a regain of interest in allochthonous strains used as probiotics for terrestrial animals or human, due to economical and regulatory constraints, but the short survival in seawater may limit application to marine fish. If viability is required, alternative treatments may be the incorporation of prebiotics in feed, and other dietary manipulations that could promote intestinal LAB. Antagonism to pathogens is the other main feature of candidate probiotics, and there are many reports concerning mainly carnobacteria and Enterococcus. Some bacteriocins were characterized, which may be of interest not only for aquaculture, but also for food preservation.