Biologically active compositions of matter comprising substituted isoquinolines and salts thereof. Certain of the compounds and their salts are novel. Acaricidal, fungicidal, herbicidal and insecticidal activity is demonstrated. In particular 3-chloro-5-acetamido-isoquinoline shows good activity as a selective pre-emergence herbicide, and 4-bromo-5-nitro-isoquinoline shows good fungicidal activity against Erysiphe graminis (wheat powdery mildew).