In microcosms, <i>Picea sitchensis</i> seedlings were grown aseptically in perlite + vermiculite with NH<SUB>4</SUB>-N or NH<SUB>4</SUB>NO<SUB>3</SUB>-N, with an ectomycorrhizal fungus, <i>Lactarius rufus</i> and/or a saprotrophic fungus <i>Hygrophoropsis aurantiaca</i>. Total and fluorescein diacetate (FDA) active hyphal length, mycorrhizal infection and plant dry weight were measured (8 to 16 weeks). In perlite with NH<SUB>4</SUB>-N, total hyphal length of extramatrical mycelium (EMM) produced by <i>L. rufus</i> was 693 m gdwt perlite or 47 m per mycorrhizal root-tip. This was reduced to 141 m gdwt in the presence of <i>H. aurantiaca</i>. Nutrient solution pH and plant growth were also reduced. In liquid culture, acidification of the medium (H<SUP>+ </SUP> equivalent) per NH<SUB>4</SUB> ion uptake was ≤ 3.8 times greater for <i>H. aurantiaca</i> than <i>L. rufus</i>. Growth (dwt) of <i>H. aurantiaca</i> was inversely related to pH. Growth of <i>L. rufus</i> was unaffected by pH 2.7 to 5.0. In perlite + vermiculite with NH_4-N, <i>L. rufus</i> hyphal length was unaffected by the presence of <i>H. aurantiaca</i>. In this experiment, no mycorrhizas formed in treatments with NH_4NO_3-N and EMM production by <i>L. rufus</i> was greatly reduced compared to treatments with NH_4-N. In a different experiment with NH_4NO_3-N, total hyphal length of EMM for <i>L. rufus</i> grown alone was apparently greatly reduced, (compared with an experiment with NH_4-N) but was increased in the presence of <i>H. aurantiaca</i>. Plant dry weight was also increased in the presence of <i>H. aurantiaca</i> with perlite + vermiculite. Total hyphal length of <i>H. aurantiaca</i> was not affected by the presence of <i>L. rufus</i> in any experiment. FDA-active hyphal length of both fungi decreased or remained constant and was 1% to 3% of total hyphal length at a week 16. Percentage mycorrhizal root-tips was similar between treatments at week 16 but differences occurred at weeks 8, 10 and 12 which suggested an inverse relationship between % infection and pH.