<ns4:p><ns4:bold>Background:</ns4:bold> Insecticide treated nets (ITNs) protect humans against bites from the <ns4:italic>Anopheles</ns4:italic> mosquito vectors that transmit malaria, thereby reducing malaria morbidity and mortality. It has been noted that ITN use leads to a switch from indoor to outdoor feeding among these vectors. It might be expected that outdoor feeding would undermine the effectiveness of ITNs that target indoors vectors, but data are limited. <ns4:bold>Methods:</ns4:bold> We linked homestead level geospatial data to clinical surveillance data at a primary healthcare facility in Kilifi County in order to map geographical heterogeneity in ITN effectiveness and observed vector feeding behaviour using landing catches and CDC light traps in seven selected areas of high and low ITN effectiveness. <ns4:bold>Results: </ns4:bold>We observed 33% and 39% visits associated with positive malaria slides among ITN users and non-ITN-users, respectively; ITN use was associated with 22% protection from malaria (crude OR = 0.78, 95% CI: 0.72, 0.84). We obtained significant modification of ITN effectiveness by geographical area (p=0.022), and identified significant hotspots using the spatial scan statistic. Most biting occurred outdoors (62%) and was by <ns4:italic>An. funestus </ns4:italic>(76%), and appeared to be more frequent in low ITN effectiveness areas compared with high ITN effectiveness areas (69% vs. 26%, p&lt;0.001), but this was due to a single outlying area.  After excluding this outlying area, outdoor biting was similar in low vs. high ITN effectiveness area (69% vs. 75%, p=0.76). <ns4:bold>Conclusion:</ns4:bold> Our data therefore do not support the hypothesis that outdoor biting undermines the effectiveness of ITNs in our study area.</ns4:p>