We report on the two-photon excited fluorescence of conjugated polymer dots (CPdots). As a new class of two-photon fluorescent probes, CPdots exhibit two-photon action cross sections as high as 2.0 x 10(5) GM, to our knowledge, the largest reported thus far for a nanoparticle. The cross section values are 3-4 orders of magnitude higher than those of conventional fluorescent dyes and an order of magnitude higher than those of inorganic quantum dots. Single particle fluorescence imaging was achieved using relatively low laser power.