Biological compounds are known to have infrared spectra indicative of specific functional groups. There is a strong interest in the use of passive means to detect airborne biological particles, such as spores and cells, which may act as biological weapons. At the sizes of interest, the infrared spectra of bacterial particles results from a combination of geometric (πd_particle > λ) and Mie (πd_particle ~ λ) scattering processes while the infrared spectrum of atmospheric particles falls into the Rayleigh limit (πd_particle << λ). In this paper we report on laboratory measurements of the infrared spectra of aerosolized Bacillus subtilis (BG) spores in air under controlled measurement conditions. Transmission measurements show an IR spectrum of the spores with features comparable to the condensed phase spectrum superimposed on a background of Mie scattering. Preliminary measurements indicate a peak extinction coefficient of approximately 1.6 × 10^-8 cm² per spore at 9.65 µm. These results are discussed in terms of their implication for passive and active infrared detection and identification of bio-aerosols.