The authors report on a study carried out to determine the experimental plague-vector efficiency of X. v. hawaiiensis compared with X. cheopis after both species had been infected with a virulent Hawaiian plague strain (S113). In deriving the numerical values for vector efficiency the concepts of Wheeler and Douglas were followed with some modifications. An additional component, the blocking-survival potential, was used to obtain a vector index.The experiments showed that the mean extrinsic incubation period was shorter in X. v. hawaiiensis than in X. cheopis but that the latter species produced more blocked fleas. The observed values for vector efficiency indicate that X. cheopis was about twice as efficient in plague transmission as the Hawaiian flea. The time and percentage mortality curves of mice dying of plague after blocked fleas had fed upon them were observed to be similar to the curves of mice succumbing to the intracutaneous inoculation of known dosages of P. pestis.In general, it was found that bacteriological culture of the faecal droppings of fleas was unreliable as a check on plague infection in fleas.