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The autofluorescence of horsetail Equisetum arvense spores excited with UV-light of 360-380 nm was studied by microspectrofluorimetry during their development from an individual cell to the formation of a multicellular thallus with the generative organs. The investigation involved the registration of the fluorescence spectra of individual intact developing cells and the measurement of the ratio of cell fluorescence intensities in the blue and red regions of the spectrum. Dry blue-fluorescing microspores showed the maxima at 460 and 530 nm and a small maximum at 680 nm. Thirty minutes after moistening in water, red-fluorescing cells arose among blue-fluorescing microspores, indicating the onset of development. Red fluorescence with a maximum at 680 nm enhanced as cells put off their cover, which brightly fluoresced in the blue region of the spectrum with the main maximum at 460 nm. By estimating the ratio of autofluorescence intensities in the blue region of the spectrum to red lightening of microspores at the first stages of development up to 24 h (in particular, their first division, the formation of nonfluorescencing rhizoid, etc.), nonviable (only blue-lightening) cells were distinguished from viable cells, in which red fluorescence began to prevail. After 25-40 days of development, the gametophyte fluorescing mainly at 680 nm formed male organs, antheridia, with blue-green-fluorescing spermatozoids. Then female generative organs archegonia with the egg cell appeared, which fluoresced blue, whereas the surrounding cells fluoresced red. It was supposed that the lightening in the blue and green regions of the spectrum is due to the presence of phenols, terpenoids, and azulenes, whereas the emission in the red region is associated with the presence of chlorophyll and azulenes. The observation of autofluorescence makes it possible to easily distinguish generative cells without additional staining.