Two generations of the cabbage root fly occurred annually, the main periods of egg laying being from the last week of May till the beginning of July, and from the third week of July till the end of August. The population of the first generation was higher and so more damaging to the crops. Predation on the egg stage by Carabid and Staphylinid beetles was confirmed to be the most important biological controlling factor of the pest. Bembidion app., T. obtusus, Atheta spp. and 0. rugosus were the principal predators. In both years predation by beetles reduced significantly (P = 0.01) the cabbage root fly egg population in the field. The reduction in the egg numbers of the root fly first generation due to predation was calculated to be 1l)I and 31 per cent in 1965 and 1966 respectively. Diazinon and chlorfenvinphos granules were applied around the base of cabbage plants on and under the soil surface. Both the diazinon treatments reduced significantly (P = 0.01) the number of Carabid predators in the field compared with the control. These losses were significantly reduced (P = 0.05) in the subsurface treatment compared with the surface treatment. The Staphylinids in the surface treatment were significantly reduced (P = 0.05) compared with both the control and the subsurface treatments, the difference between the sub-surface and the control was non-significant. The Carabids were found to be remarkably tolerant to the chlorfenvinphos insecticide. The number of Carabids captured in the treated plots did not differ from that of the control. On the other hand the Staphylinids in the surface treatment were significantly reduced at 1% level, and in the sub- surface treatment at 5, level compared with the controls. The difference between the surface and the sub- surface treatments was also significant at 56 level. Despite the reduction in beetle population by the insecticide applications, in both years the predation on the egg stage of the cabbage root fly was not significantly affected by the treatments. 51 per cent. parasitism by I. rapae and 13 per cent. by A. bilineata were recorded in 1964. This was reduced to 19 per cent. all by I. rapae in the same period in 1965 and 17 per cent. in 1966. This reduction could be due to the use of insecticides in the field. Diazinon gave a 100 per cent. control of the cabbage root fly over a period of about 11 weeks, and gave very little protection thereafter. The chlorfenvinphos was extremely active against the pest giving a 100 per cent. control throughout the season. Weather conditions were noticed to have a marked effect on the rate of oviposition by the cabbage root fly. Persistently strong wind caused a large reduction in egg -laying in the field.