Background: Allergenic diseases triggered by aeroallergens extract a health cost in quality of life and in economic impact. People generally spend 90% to 95% of their time indoors, so understanding the environmental factors that affect the presence of aeroallergens indoors are important in understanding health impact and potential intervention methods. Objective: Describe the relationship of indoor airborne pollen and fungal spores in occupied mobile homes with outdoor concentrations and other environmental factors within geographically diverse areas of Texas. Methods: Airborne pollen and fungal spores were collected during the daytime with RotoRod samplers indoor and outdoor of mobile homes in Houston and E1 Paso, Texas. Samples were counted simultaneously with a dual eyepiece microscope and identified morphologically and through staining techniques. Results: Geometric mean concentrations (counts/m3) indoors and outdoors for pollen, respectively, were Houston 7.1 and 196.4; and El Paso 17.5 and 71.5. Geometric mean concentrations (counts/m3) indoors and outdoors for spore, respectively, were Houston 98.5 and 196.4; and El Paso 36.9 and 71.5. Indoor to outdoor ratios (I/O) for pollen and fungal spores were found to be higher on average than has been previously reported. Modeling of predictive factors in Houston demonstrate that 62% and 41% of indoor levels of pollen and fungal spores, respectively, can be explained by their corresponding outdoor levels. These data suggest that the many factors associated with individual exposure to airborne pollen and fungal spores indoors are under the control of the occupant, and may additionally be influenced by the physical characteristics of mobile homes, in particular the high surface area to volume ratio and restricted flow patterns.