Hyphal extension in the filamentous actinomycete Streptomyces coelicolor A3(2) was shown to occur by addition of newly synthesized wall material in an apical extension zone. Incubation of mycelia with tritiated N-acetyl-D-glucosamine (GlcNAc), a precursor of peptidoglycan, resulted in localized incorporation of label at the apex, as indicated by light microscopic and electron microscopic autoradiography. Within the hyphal extension zone there was a sharp decrease in incorporation with increasing distance from the apex. Hyphal tip shape, examined by low-temperature scanning electron microscopy, approximated to a semi-ellipsoid of revolution and was not hemispherical. Tip shape could be represented accurately by polynomial equations of degree less than seven. The surface stress theory was successfully applied to hyphal tip growth, with tip shape related qualitatively to the inverse of surface tension within the wall of the extension zone. Surface tension was assumed to be inversely proportional to the rate of incorporation of tritiated GlcNAc. Treatment of surface-grown hyphae with beta-lactam antibiotics resulted in localized swelling of hyphal tips. Lysozyme caused swelling of tips and of other regions of hyphae, frequently giving a beaded morphology associated with septa.