PM2.5 surface concentrations were measured in smoke emitted by four wildfire events during fire seasons 2005-2008. These measurements fill a gap in the existing scientific PM2.5 observation database by providing a targeted wildfire-specific observation dataset. Four deployments occurred during various fire types including a managed-for-fuel-treatment wildfire complex, a wildfire complex, and two regional fire events. The maximum 24-h averaged values for each case were: 94.5 mu g/m(3) (2005), 425 mu g/m(3) (2006), 118 mu g/m(3) (2007), and 247 mu g/m(3) (2008). While these values are high, the diurnal concentration median and first quartile values remain below 35 and 10 mu g/m(3), respectively. For all cases, the hourly diurnal patterns exhibit peak concentrations in the mid-morning and low concentrations in the mid-afternoon. Correlations between daily area actively burning and observed PM2.5 concentrations were significant for all cases and concentration patterns were found to be similar by geographic location rather than by type of fire (single vs. region-wide). Multiple co-located monitor types, the Environmental Proof Beta Attenuation Monitor, which measures PM2.5 concentrations using a beta-beam aimed at particulates collected on filter tape, and the E-SAMPLER and DataRAM, which both use nephelometry to measure PM2.5 concentrations, showed statistically good agreement. Published by Elsevier Ltd.