Field experiments were carried out during 1988-89 at the AES-UPR Lajas substation and on a private farm in its vicinity to evaluate the efficacy of imazapyr and picloram in the control of Albizia and mesquite in two selected pastures. Both herbicides were applied by the following methods: basal bark, cut stump, frill treatment and infection. Basal bark was the most effective method for albizia control when using imazapyr at 0.237L/3.8L application; infection, the least effective. Cut stump was the most effective method when using imazapyr at 0.355L/3.8L; frill treatment, the least effective. Basal application was the most effective method when using an undiluted formulation of picloram; cut stump, the least effective. For mesquite control, cut stump was the most effective method when using imazapyr at both rates of application. Cut stump was the most effective method of picloram application; basal bark, the least effective. The mortality data of both affected species were analyzed on the basis of individual herbicide, irrespective of method of application. A significantly higher percentage of mortality of both species was registered with picloram than with imazapyr.