This is the first study of the marine microorganisms associated with Pyrosoma atlanticum, undertaken to improve understanding of pyrosome biology, the impact of pyrosome blooms on marine microbial systems, and microbial symbioses with marine animals. The diversity, relative abundance, and taxonomy of pyrosome-associated microorganisms was compared to seawater during a pyrosome bloom in the Northern California Current System using high-throughput sequencing of the 16S rRNA gene v4 hypervariable region, microscopy, and flow cytometry.