Laboratory cultures of heterotrophic protists are often xenic, meaning that the culture contains more than one microbial organism. In this study, we analyzed genome-assembly data from cultures of four marine protist flagellates—the marine malawimonad Imasa heleensis, the undescribed mantamonad strain SRT-306, the discobid Ophirina amphinema, and the cryptist Palpitomonas bilix—specifically to search for genomes of cocultured bacteria. As no external bacteria have been added to the protist stock cultures, it is probable that the cocultured bacteria came from the original water samples from which the protists were isolated. At least some of these bacteria are consumed as a food source by the protists, all of which are obligate heterotrophs. From four separate metagenomic de novo assemblies for these mixed cultures, we identified 28 scaffolds, which BUSCO analyses suggest represent complete or near-complete bacterial genomes. These scaffolds range in length from 3,139,436 to 6,090,282 bp and encode 2873 to 5666 genes. Only eight of the 28 scaffolds corresponded to entries in the NCBI genome database, meaning that 20 of these scaffolds represent genomes from putatively novel bacterial species. Our findings highlight that data like these, which are often discarded or overlooked, can be a source of novel genomes and/or species.