Recent studies have suggested that intracellular Wolbachia bacteria are necessary for reproduction and survival of adult filarial worms. We now report results of in vitro studies of effects of antibacterial antibiotics (tetracycline, rifampicin, chloramphenicol, azithromycin, and doxycycline) on Brugia malayi infective larvae (L3) motility and molting. All of the antibiotics tested except chloramphenicol decreased L3 motility by 50% or more at 10 days, with minimal effective concentrations (MECs) of 20-100 μg/ml. Tetracyclines, rifampicin, and chloramphenicol inhibited L3 to L4 molting by 12 days in a concentration- and time-dependent manner, with MECs in the range of 1-20 μg/ml. These studies show that antibiotics active against Rickettsiaceae inhibit B. malayi L3 molting at low concentrations in vitro; higher concentrations kill the larvae. While it is possible that antibiotics directly affect filarial L3, we believe it is more likely that the effects seen are indirect effects related to bacterial killing. Index Descriptors and Abbreviations: Brugia malayi; Filarioidea; Rickettsiaceae; Wolbachia; antibiotics; NI-medium; NCTC-135 and IMDM, Iscove's Modified Dulbecco's Medium; FCS, fetal calf serum; Tet, tetracycline; Rif, rifampicin; Chlor, chloramphenicol; Azith, azithromycin; Doxy, doxycycline; IM, incomplete molting larvae; MEC, minimum effective concentration; PCR, polymerase chain reaction.