The antimicrobial activity of beta-hydroxynaphthaldehyde (HNA) was tested against 40 different fungal species isolated from deteriorated cotton, textiles, canvas, paper, book bindings, photographic films, leather, footwear and other similar articles. The concentrations 10-100 mug/ml were tested by poisoned food technique and 60 mug/ml (0.006%) was found to render 100% inhibition in fungal growth. Further investigations on mode of action were carried out using <i>Aspergillus flavus</i> as a representative fungus. The observations on radial growth revealed a gradual decrease from 13.0 to 4.2 mm at 10.0-50.0 mu/ml concentrations and 28°± 1°C. Dry weight of mycelium also decreased from 496 to 28.0 mg at the same dosage. After 7 days' incubation no growth was seen at 40 and 50 mug/ml concentrations but after 12 and 15 days visible growth occurred. The effect of HNA on the fungal morphology was loss of colony color, reduction in head size, shortening and thickening of the mycelium and complete inhibition of sporulation at 50 mug/ml. The results on the secretion of extra-cellular enzymes, viz. amylase, cellulase, protease and lipase, indicated quantitative reduction with the increasing concentrations of HNA. No secretion was found at 40 and 50 mug/ml. This compound was highly effective and possibly behaves as a general cytoplasmic poison. The antimicrobial properties were considered to be due to a highly reactive aldehyde group attached in the beta-position in the naphthalene ring. It has been recommended as a long-term protectant against biodeterioration of most of the materials.