Aerial delivery of 1080 (sodium monofluoroacetate) baits is the main technique for reducing populations of New Zealand's foremost vertebrate pest, the Australian brushtail possum, in large areas of inaccesible country. Surveys after pilot-controlled aerial sowing of baits in seven operations in forests showed that inaccurate navigation along the swaths left up to half the target zone untreated. Kill was estimated to average 75%. Inadequate coverage with baits was therefore believed to be a major factor in the survival of possums during aerial control operations. This was confirmed in field trials using rhodamine B as a biomarker to reveal acceptance of non-toxic baits. More possums were unmarked in partially treated blocks than in completely treated blocks. After a large-scale aerial control operation, proportionally more possums survived in untreated gaps than in treated areas. Six operations that used navigation guidance systems (Decca Flying Flagman and GPS) yielded complete coverage and high levels of kill (mean of 92%) in five. Precision sowing of possum baits prevents survival of possums by failure to encounter baits, and enables lower rates of bait application. This will give large cost savings and improved environmental safety. A small proportion of a population may still not be targeted because of individual dislike of bait or failure to encounter baits because animals stayed in the forest canopy during operations. Development of more palatable and longer lived baits may facilitate local extermination of possums.