Recently, sweet basil (Ocimum basilicum L.) grown in greenhouses was introduced in Almería, Spain. It is typically cultivated in soil or perlite soilless culture. During the last 6 years, the following symptoms were observed sequentially in basil cultivation: yellowing and wilting of apical tips, wilting and necrosis of leaves and petioles, stunting, black lines along stems and petioles, and discoloration and necrosis of the xylem. Symptoms began at the apex and progressed to the plant base. Within 4 months of planting, symptoms developed in more than 14% of soil-cultivated plants, and in more than 13% of perlite-cultivated plants. Isolations from diseased xylem revealed the presence of Fusarium oxysporum. Inoculations were performed with a fungal suspension (10⁴ CFU/ml) on basil cv. Genovesa, by drenching plants grown in sterile substrate or dipping the roots and transplanting plants into sterile substrate. Of 30 isolates, 80% were pathogenic and resulted in symptoms of the disease described above. The pathogen was reisolated from all inoculated plants. Inoculating Melissa officinalis L., Salvia officinalis L., Origanum majorana L., Mentha piperita L., Satureja hortensis L., and Thymus vulgaris L tested specificity of F. oxysporum. This test utilized the same methods used for basil. None of these species developed symptoms. Results indicated that symptoms of the disease on basil were caused by F. oxysporum f. sp. basilici. Since cultivation of basil is relatively new to Almería, it was necessary to determine the source of the inoculum. Accordingly, 3,200 seeds from Germany and Italy, the primary source of seed in Almería, were analyzed. F. oxysporum was isolated from 0.5% of the seeds. Following methods used earlier, all isolates were inoculated on basil. Fifty percent of the isolates reproduced the disease symptoms. The results suggest that the seeds from Germany and Italy were the source of the inoculum, and to our knowledge, introduced the disease into the growing basil cultures of Almería.