The influence of particulate organic material (POM) and the contribution of biomass smoke on air quality and visibility remain a paramount issue in addressing regional haze concerns in US national parks. Measurements during the Yosemite Aerosol Characterization Study (July–September 2002) indicated an aerosol dominated by POM (∼70% of identified species) and strongly influenced by biomass smoke. Here we report aerosol size hygroscopic growth measurements for dry (RH<5%) aerosol diameters of 100 and 200 nm as measured with a controlled relative humidity tandem differential mobility analyzer. Hygroscopic growth was found to be negligible for relative humidity (RH) <∼40% within the sensitivity of the method. For RH>40%, particle size typically increased smoothly with RH, and overall hygroscopic growth at high RH was low to moderate in comparison to the range of values reported in the literature. For RH>80%, both monomodal and bimodal growth profiles were observed during the study, with 200 nm particles more often splitting into bimodal profiles (68% of cases), indicating some degree of external mixing. Trimodal growth profiles were observed on two occasions during periods of changing meteorology and aerosol composition. For bimodal profiles for 200 nm dry particles, particle diameter growth factors at RH=80% (D(RH=80%)/Do where Do is measured at RH<5%) were 1.11±0.04 and 1.29±0.08 for the more and less hygroscopic modes, respectively. Ensemble D/Do was calculated using a cubic-weighted sum of D/Do of individual modes. For 200 nm particles, average ensemble D(RH=80%)/Do was 1.15±0.05 and D(RH=90%)/Do was 1.31±0.06, and were slightly large for 100 nm particles. These growth factors are dramatically lower than those for typical ambient aerosol ionic components such as sulfate, nitrate and sodium salts and sulfuric acid. An inverse relationship between the POM fraction of PM2.5 and hygroscopicity was particularly evident for 200 nm particles with D(RH=80%)/Do approaching ∼1.1 as the POM/ionic mass ratio exceeded 10. Linear correlations with several measurements of POM and select biomass smoke markers were −0.74<R<−0.59 for 200 nm and −0.43<R<−0.25 for 100 nm particles. The limited hygroscopic growth of the carbon dominated, biomass smoke influenced aerosol at Yosemite has important implications to assessing the role of POM in visibility degradation in national parks.