We analysed the gut microflora of the Long-Jawed Mudsucker, Gillichthys mirabilis by polymerase chain reaction/denaturing gradient gel electrophoresis and cloning and sequencing 16S rRNA gene amplicons. Fish were collected at five sites in northern and southern California, USA. The gut microflora assemblages of all G. mirabilis were similar, very simple and dominated by one or more Mycoplasma ribotypes. Hindguts were dominated by Mycoplasmas that were most similar to a ribotype retrieved from Atlantic salmon guts. A Mycoplasma ribotype that was 95% similar to Mycoplasma mobile was the dominant in the foregut.