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How charge carriers are distributed in a bulk heterojunction (BHJ) under illumination is central to the understanding of organic photovoltics and photodetectors. Here, we apply nonlinear optical microscopy to quantitatively map the spatial distributions of electric fields in two lateral organic BHJs: poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (PCBM) and poly(4,4-dioctyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) and PCBM. For the former, we observe the development with time of a depletion region adjacent to the electron-collecting electrode. In the latter, the device is stable and characterized by a nearly linear potential drop. We discuss the origins of field distributions and space charge accumulation in organic BHJs.