Previously described concepts of inoculum potential (IP) and methods to measure the IP of arbuscular mycorrhizal fungi (AMF) are reviewed. The concept of IP is elusive and needs developing- A new definition of IP is described here us the number of viable fungal propagules and structures coupled with their initial infectivity, and the IP can be calculated from the formula IP =(N × H × K)+S or IP = (N × L) +S, where N= numbers of vesicles in roots and points of hyphae connected with the roots per unit length of the roots; W= root weight; K= root length per unit weight of the roots; L= root length: S= numbers of viable spores in an inoculum. For the AMF that product spores in roots, few spores, or no spares, the formula becomes IP =N × W × K = N × L. The parameter of the IP is therefore the total numbers of vesicles and/or spores in roots, points of hyphae connected with roots, and viable spores in an inoculum of any type. The correlation coefficients between the IP of the inoculum of Glamus Mossear (Nicol. &Gerd.) Gerdemann &Trappe, Glomus versiforme (Karsten) Berth,or Sclerocystis sinuosa Gerdemann &Bakhi and the amount of early stage infection produced by the corresponding were all > 0.93 (P < 0.01)on three host plants. Gassypium hirsutum L., or Sarghum Sudanense L., It is suggested that the IP should be easily and rapidly estimated with accuracy by the present method.