Trimethoprimium-phosphomolybdata, (TM)3PM, was used as an ion-exchanger in the construction of a TM-responsive plastic membrane electrode. The calibration graph slope of the electrode is 56.5 mV per TM concentration decade when preconditioned by soaking in water, over a concentration range of 2.3 x 10(-6)-10(-2) M with instantaneous response. The electrode has a life span of 24 h when soaked in 10(-3) M TM solution and 5 days when soaked in distilled water. The working pH and temperature ranges of the electrode are 1.6-6.2 and 30-65-degrees-C, respectively. The isothermal coefficient of the electrode is 0.00091 V-degree-C-1. The electrode is highly selective for TM towards inorganic cations, sugars and amino acids. It can be employed as a potentiometric sensor for determination of TM in pure solution and in tables.