Two studies using three ruminally cannulated beef steers in the first one fed a roughage diet (98% chopped alfalfa at maintenance) and 12 beef steers in the second fed limited ration (85% adlib intake) composed of 90% concentrate diet (80% cracked corn, 10% soybean meal), and 10% chopped alfalfa were allotted to treatment groups receiving a supplement of either monensin / tylan (m27.5 ppm / 11 ppm), lasalocid (33 ppm) or a daily rotation of the two ionophores to examine the effect of the ionophores on rumen population of anaerobic fungi over time and the relationship between fungal numbers and methane production. In the first study, nylon bags containing leaf blades of 5mm length were incubated for 24 hrs in the rumen. Samples were counted on days 0, 1, 3, 5, 7, 11, 14, 16, 21, 22, 23, 24, and 28 Fungi sporangia on blades were counted microscopically (00x). Study 2, steers were assigned as above and after methane production was measured using indirect respiration calorimetry, samples of rumen contents were collected by stomach tube pretreatment, and on days 2, S, 16, and 45 after initiation of ionophore treatments. Differences of fungi numbers over time and between treatments were not significant, yet, there was a trend of initial ionophore inhibition on fungi with limited evidence of adaptation over time there was a positive correlation (P < 0.05) between fungi numbers and methane production. These results indicate a fungal/methanogen interaction.