Many particles captured in water bottles settle below the sampling spigots and escape detection when the sampler design does not allow for removal of all the enclosed water. Particles lost from 30‐liter Niskin bottles can increase the measured total concentration of particulate matter by a factor of 1.1–1.7 in open ocean samples. Large numbers of particles as small as 4 µm are lost. Scanning electron microscopy showed a significant loss of diatoms, Acantharia, Foraminifera, coccospheres, dinoflagellates, fecal pellets, organic matter, carapaces, and aggregates of particles.