SUMRlARYTwelve phages isolated from sewage and active on lactobacilli were examined by electron microscopy with a negative-staining technique, Those phages active on Lactobacillus fermenti (heterofermentative) possess icosahedral heads and sheathed tails which end in base-plates and pins. Those phages active on Lactobacillus casei (homofermentative) differ in that their heads are octahedral or icosahedral and they possess collars. The overall length of all the phages is similar and their base-plates remain attached to the sheaths when these contract. No tail fibres were seen. A temperate Lactobacillus fermenti phage was also examined. It has a small hexagonal head and a long unsheathed tail which ends in a starshaped structure.