The May 18, 1980, eruption of Mount St. Helens, Washington, produced both white and gray pumice, similar in composition but varying in phenocryst, microlite, and vesicle content. The white pumice has fewer phenocrysts, no microlites, and higher vesicularity, and is thus less dense than the gray. In addition, vesicles in the white pumice are larger and more interconnected than those in the gray. Both white and gray have effective (crystal-free) vesicularities (85.7% and 72.2%, respectively), close to the traditionally accepted "fragmentation vesicularity" of 75%-77%. Given that the two pumice types were erupted concurrently for several phases of the eruption and therefore probably had similar eruption paths, we suggest that differences in vesicularity and vesicle size distributions result from the presence or absence of microlites. The presence of microlites in the gray pumice not only increased the magma viscosity and effective vesicularity, but appears to have aided bubble nucleation and hindered bubble expansion and coalescence. Thus, magmas with microlites may fragment at a lower bulk vesicularity than those without microlites. Fragmented microlite-bearing clasts are also likely to expand less after fragmentation and therefore more closely preserve the bubble distribution and structure at the time of magma fragmentation.