We hypothesized that the optical plankton counter (OPC) senses particles in situ that are not collected by nets and analyzed in the laboratory. An OPC was deployed in situ between 1998 and 2004 in the mouth of a bongo net with 505-¿m-mesh nets in the upper 210 m at stations in the California Current Region. Here we compare paired data sets from the OPC in situ and the OPC analysis in the laboratory of the simultaneously collected net samples for four seasons of 2 years. We restricted our analysis to particle sizes 1.26--6.35 mm equivalent spherical diameter (ESD), a size class shown from the lab OPC data to be retained efficiently by the net. On average, 4 (3) times more particles by number (volume) were sensed in situ by the OPC than sensed in net collections by the OPC in the lab. These values varied an order of magnitude among the eight cruises examined. Time of day, distance offshore, season, year, chlorophyll a concentration, and Brunt-V¿is¿l¿ frequency each explained significant variation in these differences. The excess of particles sensed in situ over that measured in the net samples was due primarily to smaller particles in the 1.26--6.35 mm ESD range. We infer that the particles measured by the OPC in situ but not in the lab were fragile and thus not collected by the net. We hypothesize that these fragile particles are primarily aggregates and abandoned houses of larvaceans.