low humidity adversely affected the hatching process but not the rate of embryonic development of eggs of the closely related Ophionyssus natricis (Gervais). Similar information for the northern fowl mite is not available. II. Effect of constant relative hum dity on the li of northern fowl mite e gs.* Very low humidity caused only a small decrease in egg hatch (74.2% hatch at 20% relative humidity). Thus, the effect of relative humidity on the hatching percentage was less marked than that of temperature (no hatch at 40 C). The effect of relative humidity on the incubation period of eggs of parasitic mites has been studied little. Changes in relative humidity did not seem to affect the incubation period of northern fowl mites (Combs and Lancaster, 1965, loc. cit.) or Ornithonyssus bacoti (Hirst) (Skaliy and Hayes, 1949, American Journal of Tropical Medicine 29: 759772), at least at optimal temperatures. Thus, under the conditions of the present experimental study, temperature was probably the chief factor influencing embryonic development.