The effects of surfactant type and concentration on the particle size and electrophoretic mobility of cross-linked poly(N-isopropylacrylamide) (polyNIPAM) microgel latexes were studied. Without surfactant the particle diameter of the polyNIPAM decreased by a factor of 2 when the temperature was increased above 31-degrees-C, the cloud point temperature (CPT) for polyNIPAM homopolymer in water. Anionic sodium dodecyl sulfate (SDS) in the concentration range 0.001-0.008 M caused the particle diameter to increase and the CPT to shift to higher temperatures. Higher SDS concentrations caused further swelling of the latex and the diameters became nearly independent of temperature in the range 10-60-degrees-C. Nonionic Triton X-100 had no effect on the particle diameter/temperature behavior indicating no surfactant binding. Below the CPT cationic dodecyltrimethylammonium bromide had no effect on swelling, whereas the cationic surfactant induced latex coagulation at the CPT. Surfactant binding was also reflected in the electrophoretic mobilities. Low levels of SDS binding gave higher negative mobilities which were temperature sensitive. High SDS concentrations removed the temperature swelling transition so mobility was temperature insensitive. Cationic surfactant binding increased the mobility of positively charged polyNIPAM latex; however, in contrast to SDS, the mobility was temperature sensitive at high surfactant concentrations.