Egg laying of the two-spotted spider mite, Tetranychus telarius (L.), was found to be inhibited by fairly low concentrations of a number of antibiotics. Although data are not given, similar results were achieved with the European red mite, Panonychus ulmi (Koch). Listed in the apparent order of their effectiveness, these antibiotics were: cycloheximide, streptovitacin A, antimycin A, hygromycin B, and amphotericin B. Amphotericin A and several derivatives of cycloheximide were also fairly effective in suppressing egg laying of both species. Cytovirin was as effective against the European red mite as previously reported against the two-spotted spider mite. At the higher concentrations, especially with antimycin A, the antibiotics also caused some mortality of the adult mites. The antibiotics seem to interfere with some process in the division and growth of the egg cells. The more active materials were phytotoxic and especially injurious to apple and pear trees. Cycloheximide at 50 and 100 p.p.m. gave good control of the two-spotted spider mite in field tests on mature peach trees without appreciable injury to the foliage. The more active antibiotics were obviously systemic in the leaves. Cycloheximide was not systemic in soil treatments and in spray oil or ethylene glycol was not appreciably translocated to the leaves when painted on the trunks and basal limbs. Several antibiotics, including cycloheximide, moved to the leaves when injected into the trunks of peach seedlings in the greenhouse. Cyelohximide, when used with different surfactants or added to some of the more common insecticide sprays, was also active in suppressing egg laying of both species of mite.