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The relation between moisture content of oak sapwood and rate of development of Lyctus brunneus has been investigated at 23d̀C. Oviposition took place normally in samples of various moisture contents between 5.5 and 23.5%. The eggs hatched in wood of 7.7% moisture content, but not of 61% moisture content, although the embryos developed fully. The rate of gain in weight of young larvae increased to a maximum as the moisture content of the wood rose to about 14.2% and then fell again. The rate of development from egg to adult followed a somewhat similar curve with a maximum at 16% moisture content. At all moisture contents the first male emerged a few days earlier than the first female. The average weight of the male beetles was less and their moisture content greater than those of the corresponding females. The average weight of the beetles showed a tendency to increase at moisture contents of the wood above 97%. The average moisture content of the males was fairly constant up to 156% moisture content of the wood, but that of the females rose slowly as the moisture content of the samples increased. In wood of 15.6% moisture content at 23d̀C. the minimum period of development from egg to adult was 167 days; at 14.2% moisture content and 27.5d̀ C. this was reduced to 142 days. Green timber was shown to be liable to infestation as soon as a superficial skin was formed of a suitable moisture content; the upper limiting moisture content is probably the fibre saturation point. Control of moisture content of timber cannot be achieved sufficiently easily for it to become an important method for the control of Lyctus infestation.