Infestations of the rice gall midge Pachydiplosis oryzae (Wood-Mason) in Ceylon were studied in 1968. The three larval instars, the prepupa and pupa are described. Total development time is 18–22 days. Dissection of infested rice seedlings showed that first-instar larvae move in 6–12 h from the leaves where they hatch to the shoot apices to which they are specifically attracted, without boring into plant tissues. Larvae feed at the base of the growth cone throughout their development; if more than one larva reaches a shoot apex only one individual survives to maturity. Gall formation results from suppression of the growth cone, development of radial ridges from the innermost leaf primordium just above the level of the posterior end of the larva, and then an elongation of the leaf sheath. Death of the larva, achieved experimentally by means of diazinon or fenitrothion applied to the soil, up to and including instar 2 was followed by renewed activity of the growth cone, but after the same treatment during instar 3 the growth cone was not re-activated. The development of the gall is seen as being due to diversion of nutrients from the growth cone to the midge larva, and possibly by substances produced by the first-instar larva and prepupa which stimulate growth of the radial ridges and gall elongation, respectively. Firstinstar larvae develop only in active shoot apices; thus development proceeds normally in the terminal apex, but is inhibited in the inactive axillary shoot apices. Under high population pressure all shoot apices become infested, but larval dormancy results in staggered gall formation and adult emergence. Larval dormancy in inactive axillary shoot apices may explain seasonal carry-over of the pest.