Two methane monooxygenase (MMO) systems have been identified in methanotrophic bacteria, namely, a soluble or cytoplasmic MMO and a membrane-associated or particulate MMO. The active site of the well-characterized soluble MMO contains a bis-14-hydroxo-bridged diiron cluster. Xray crystallographic studies of the particulate enzyme, pMMO, have identified two copper centers on the alpha subunit (pmoB) of the alpha beta gamma trimer and a site at the interface of the beta gamma subunits filled by a Zn, apparently from the crystallization buffer. In our hands, pMMO preparations containing 1-2 iron atoms per alpha beta gamma show the highest catalytic activity. We have employed Mossbauer spectroscopy to characterize the iron in our preparations, Interestingly, we find in pMMO a component with the same spectral properties as the antiferromagnetically coupled diiron(III) cluster in the soluble enzyme. In whole cells, we find nearly 1 diiron center per alpha beta gamma of pMMO; in purified enzyme preparations, only 10% of the sites appear to be occupied. These occupancies correlate well with the measured specific activities of purified pMMO and pMMO in whole cells. We suggest that it is the "Zn site" that accommodates the diiron center in active pMMO.