SummaryDead workerBombus honorumand queenB. terrestrisbumble bees representing the narrowest elongate head and the broadest short head likely to be present in the local bumble bee population, were mounted as probes with their proboscides folded in the non-feeding position. The probes were inserted into the corollae of 60 plant species and the distance from the distal tip of the labrum to the bottom of the functional corolla tube, or spur, was recorded at each insertion. The measurements gave a minimum and maximum‘exclusive corolla length’—the part of the corolla tube that can only be probed by the proboscis, because it cannot be entered by the bee's body. The mean exclusive corolla length varied from 0·8 mm forAnthriscus sylvestristo 18·9 mm forMimulus guttatusin the species studied. The technique is intended to facilitate the interspecific collation of corolla lengths, proboscis lengths and bee sizes in studies of food resource availability.