Biomass combustion in cookstoves has a substantial impact on human health, affects CO2 levels in the atmosphere, and black carbon (BC) and organic carbon (OC) affect the earth's radiative balance. Various initiatives propose to replace traditional fires with "improved" (nontraditional) cookstoves to offset negative local and global effects. In this laboratory study, we compared the size, composition, and morphology of ultrafine particulate emissions from a "three-stone" traditional fire to those from two improved stove designs (one "rocket", one "gasifier"). Measurement tools included a scanning mobility particle sizer, PTFE and quartz filter samples, and transmission electron microscopy. In the improved stoves, particulate mass (PM) emissions factors were much lower although median particle size was also lower: 35 and 24 nm for the rocket and gasifier, respectively, vs 61 nm for the three-stone fire. Particles from improved stoves formed clearly defined chain agglomerates and independent spheres with little evidence of volatile matter and had a higher proportion of BC to total PM, although overall BC emissions factors were fairly uniform. The 3-fold increase in quantities of sub-30 nm particles from improved cookstoves warrants further consideration by health scientists, with due consideration to the higher combustion efficiencies of improved cookstoves.