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The work described in this thesis is concerned with two parasitic helminths, Fasciola hepatica and Ostertagia ostertagi. Although the major part of the thesis describes investigation into fascioliasis in both cattle and sheep, the first section records a hitherto undescribed condition in cattle where both F. hepatica and O. ostertagi coexisted in significant numbers. Single experimental onfections of cattle and sheep with metacercariae of F. hepatica were studied, and these were followed by observations on the sequential development of an outbreak of fascioliasis in sheep grazing under natural conditions, a subject investigated in detail for the first time. The final section describes a method of control by use of a fasciolicide. The thesis is divided into five sections as follows: Section I Field Studies on Clinical Parasitism in Young Dairy Cattle in South-west Scotland. The first part of this section reviews a series of out breaks of ostertagiasis occurring during the course of the winter (Type II). This is followed by the description of a further series of outbreaks of parasitic disease in which both O. ostertagi and F. hepatica were present in significant numbers (the fascioliasis/ostertagiasis complex). The two conditions had different background histories, seasonal incidences, clinical signs and necropsy findings, and these differences are discussed in detail. Section II Experimental Fasciola hepatica Infections in Calves. Single oral inoculations of susceptible calves with 1000 or 2000 metacercariae of F. hepatica resulted in the establishment of an adult fluke burden capable of producing clinical signs and death. The haematological and blood biochemical values recorded at weekly intervals demonstrated the presense of a macrocytic, normochromic anaemia and a hypoalbuminaemia. At post-mortem all but two of the infected calves showed a percentage take (i.e. the percentage of the inoculation which became established) of between 16% and 37%; the two remaining calves, given the higher level infection, showed very low percentage takes at necropsy. Section III Experimental Fasciola hepatica Infections in Sheep Inoculation of susceptible lambs with single oral doses of 1000 metacercariae of F. hepatica each resulted in the development of clinical fascioliasis followed by death between 12 and 23 weeks after infection. A severe, macrocytic, hypochromic anaemia was recorded commencing 5 to 6 weeks post-infection, the degree of anaemia being proportional to the number of flukes recovered at necropsy. Reticulocytes appeared in the peripheral circulation and a hypoalbuminaemia was also present. The percentage of the inoculum established ranged from 10.8% to 81.5%. An outbreak of clinical fascioliasis, in lambs grazing permanent pasture known to have been responsible for outbreaks of fascioliasis in sheep during the previous two years, commenced during the first part of October. The major clinical signs were weight loss and pallor of visible mucous membranes; a macrocytic, hypochromic anaemia developed with reticulocytes present in the peripheral circulation. A hypoalbuminaemia and in many cases a frank hypoproteinaemia was recorded in these lambs. The total number of F. hepatica recovered from individual animals at necropsy ranged from 110 to 1628. Concurrent studies on the changes in pasture populations of metacercariae of F. hepatica revealed that the maximum number of metacercariae was available between mid-August and mid-September although metacercariae were available to a varying degree at all times of the year. Section V The Use of Nitroxynil in Ovine Fascioliasis The efficiency of nitroxnil administered in the course of an outbreak of fascioliasis was studied. At a dosage rate of 10 mg. per kg. bodyweight, the drug arrested mortality and resulted in rapid improvement of the animals' general condition with the anaemia disappearinh within 3 weeks of treatment. F. hepatica eggs disappeared from the animals' faeces within 5 days of treatment and they were not present again until 9 weeks post-treatment athough is was 13 weeks after treatment before the majority of the lambs had positive faecal egg counts.