A comparison was made between 14-day-old populations of Nematodirus battus from single infections and from those which developed concurrently with Haemonchus contortus from the day of infection and from days 5 and 10 after infection. The numbers of Nematodirus fell with decreasing length of exposure to Haemonchus and were negatively correlated with the numbers of Haemonchus. The percentage of the populations of Nematodirus in the adult stage and the mean worm lengths fell with increasing length of development with Haemonchus, the lowest values being found in infections which developed concurrently with the abomasal worm for the full 14 days (days 0–14). Mean Nematodirus worm lengths were negatively related to abomasal pH and Na+ levels in populations which were concurrent with Haemonchus for the last 9 days (days 5–14) and for the full 14 days of development studied. Nematodirus egg capacity also fell with exposure to Haemonchus, but was lower in the worm populations exposed for the last 9 days than in those exposed for 14 days. Egg capacity was related to abomasal electrolyte levels in populations which had developed concurrently with Haemonchus for the last 9 days and for the full 14 days. Significant differences between the population parameters of Nematodirus from the proximal and middle third of the small intestine were only found in the single Nematodirus infections and in those which were concurrent with Haemonchus for the last 4 days (days 10–14). The differences in the populations of Nematodirus were related to the probable changes in alimentary conditions affected by the different periods of concurrent infections of Haemonchus. It was suggested that the alimentary changes may have caused the loss of adult Nematodirus and have influenced the growth, development and reproductive capacity of the intestinal worm by acting upon the fourthand fifth-stage larvae.