The consortium of the colonial protozoan Solenicola setigera and the frustule of the diatom Leptocylindrus mediterraneus is common and wide-spread in the Pacific Ocean between 41°N and 34°S. In the oligotrophic waters of the open-ocean, the consortia tend to appear near the deep chlorophyll maxima with a low abundance (< 20 frustules l-1). These Solenicola–Leptocylindrus consortia comprised single or a few frustules, with the protozoan colony restricted to the central section of the frustule. In more eutrophic waters, the consortia reached densities up to 8000 frustules l-1, with chains of up to 2000 μm length and with a high percentage of the protozoan colonies spread along the entire frustule. No free-living specimens of L. mediterraneus were observed in this study. The frustule was devoid of cell contents and exhibited an abnormal morphology in its central section with the occurrence of certain structures, namely the “convex walls” and “growth bands.” The morphology of protozoan cells attached to the growth bands was highly flattened and differed from that in the rest of the diatom frustule. These features suggest that the protozoan maybe able to control the growth of the frustule, used by it as a substrate.