Homozygotes die as larvae under standard rearing conditions but under uncrowded conditions, some survive to the pupal stage. Rfx49/Rfx253 larvae show no sensitivity to either an attractive (butanol) or repulsive (n-octyl-acetate) odour, in contrast to wild type. Rfx49/Rfx253 larvae do not avoid high concentrations of NaCl, in contrast to wild type. Rfx49/Rfx253 larvae show normal phototaxis behaviour. Rfx49/Rfx253 and Rfx253/Df(3R)S143702 adult escapers have a characteristic uncoordinated phenotype; the flies cannot stand or walk, legs are crossed and wings are held up. The flies react to light stimulation by moving their legs and wings in an effort to right themselves, but they are limited to a few steps or flip from one side to another. The average resting transepithelial potential (TEP) of macrochaetae (recordings have been performed on either the posterior dorsocentral or anterior scutellar bristle) in Rfx49/Rfx253 adults is significantly lower than in control siblings. No mechanically elicited changes in TEP are detected in the mutant bristles. No receptor currents are elicited by mechanical stimulation in the mutant bristles. The resistance at mutant bristles is on average, one-third of that at control bristles. No sound-elicited potentials are detected in the antennal nerve of Rfx49/Rfx253 adults, in contrast to wild type. Rfx49/Rfx253 adults show defects in the wing campaniform sensilla; the dendrite is never situated under the dome-shaped sensillum (as occurs in wild type) and sometimes ends well before the sensillum. In most cases, the cilium is shorter than normal or absent. Rfx253/Df(3R)S143702 adults show defects in the chordotonal organ of the femur; the cilium is either absent or shorter than normal and distinct subregions of the cilium cannot be distinguished, in contrast to wild type. The dendritic transition zone appears swollen. Rfx49/Rfx253 adults show no gross defects in Johnston's organ, but the ultrastructure of individual scolopidia is profoundly altered; no typical cilia are seen and the anchoring axial filament appears to be missing. The scolopale rods that enclose the cilium are not as regularly arrayed as in wild-type flies. The dendritic cap appears normal.