Rattus norvegicusoccurs in Singapore, with several other commensal rodent species. This preliminary study examined populations and a possible control method in urban and semi-urban areas. Commensal rats are generally undesirable, and a possible role forR. norvegicusin spreading any introduced plague is a particular worry. Populations were examined by catch, mark, release (CMR) and by removal (regression) methods. Species and numbers indicated by removal were—R. norvegicus(57.4, 31.9 and 48.5/ha in different trials),R. exulans(37.2/ha) andMus musculus(8.3/ha). There were indications that these figures were very considerable underestimates; a Lincoln Index, using recovered dead rats (after poisoning in a CMR Plot) as the second sample, estimatedR. norvegicusat 985/ha (i.e. in a ca 0.6 ha plot—618 ± 694); the recovery of only three out of 32 marked rats, and the presence of many dead but uncollected rats, even though more than the number estimated by removal was picked up in the plot (58) Banana, coconut and dried fish cage baits all trapped rats. Maize cakes were not worth the extra labour. Analysis of catch by trap position suggested thatR. exulansis more common upon roofs,R. norvegicuswas absent from a semi-rural environment in whichR. r. diardiiwas common. In a control programme, wax-bound maize-based anticoagulant baits were used—distributed at 91/ha, replaced, at 2—4 day intervals if accepted, to total nine rounds and 818 baits/ha at a cost of M$21.00. Acceptance of baits was high initially, and further trapping showed that reasonable control had been obtained. This method seems more appropriate to the conditions than the permanent baiting stations used in temperate areas, because of the lack of discrete points in sewers etc., the disrupting influence of frequent tropical storms, rapid deterioration of baits, and competition from other animals, particularly ants. Drawbacks include the danger to non-target animals from the baits and the temporary stench of dead rats.