Besides nitrogen, potassium (K) is a critical nutrient for plants. In most soils, 90 to 98% of the total K in the rooting zone is relatively unavailable, fixed in minerals such as micas or feldspars (Sparks and Huang, 1985) compared to exchangeable K, that ranges from 0.1 to 3%. The microbial K pool is estimated to be 0.25 to 0.5% of the total K (Scheffer, 2002) but little information is available on its role in K dynamics. To study K cycling and the role microorganisms in K transformations, a reliable measurement of microbial biomass K is needed. In this study, we developed a microbial K test based on a chloroform fumigation-extraction method. An optimal post-fumigation extractant was identified that maximized extraction efficiency in fumigated samples but minimized extraction of K from non-fumigated control soils. An extraction conversion factor was developed by inoculating a range of soils that varied widely in texture with mixed fungal and bacterial suspensions. Our results showed that a significant amount (up to 30%) of the exchangeable K is immobilized in the microbial biomass. This suggests that microbial biomass K could play an important role for plant nutrition which has been overlooked in the past relative to K cycling in soils.