We have studied the occurrence of a genus of allergic interest, Alternaria, in the outdoor and indoor atmosphere of 14 homes in Córdoba (Spain). The sampling was carried out by the method of sedimentation on a mycological medium twice a month for a period of one year. We detected an overall 3955 colonies of Alternaria belonging to seven different species of this genus (A . consortiale, A . crassa, A . dendritica, A . harzii, A . japonica, A.tenuis and A . tenuissima), some of which are proven allergenic agents. A . tenuis was by far the most frequent of the above mentioned species, both indoors and outdoors. Three of the species identified (A . consortiale, A . crassa and A . harzii) occurred in greater concentrations indoors than outdoors; the remaining four were found to occur preferentially outdoors.