The malic dehydrogenase activity was determined by the modified method of Ochoa (1955) using tissue homogenates of various parasitic helminths. Worm parasites were mostly collected from local abattoir, and removed from the organ or tissues of the naturally infected animal hosts, and some materials were also obtained from the human hosts. The helminths used in this experiment include 3 kinds of nematodes, 5 kinds of trematodes, and 8 kinds of cestodes. They were throughly washed and homogenized in glass tissue grinder in ice chilled water bath, and then centrifuged. The supernatants were designated as enzyme preparations. The hydrogen concentrations of buffer solution were pH 1.4, 2.7, 3.5, 4.2, 5.2, 7.4, 8.2, 9.3, 10.2, 11.6, and enzymatic reaction of this experiment was performed at incubation temperature of 20, 30, 40, and 50 degrees C. The extinction of Nicotinamide Adenosine Dinucleotide (NAD) was measured by spectrophotometry at the wave length of 340 millimicron. The results of the experiment were as follows: 1. The malic dehydrogenase activity occurred over all kinds of parasitic helminths used in this study. And the activity on sparganum turned out to be highest. 2. All helminths displayed their maximum activity in the range of alkaline pH. 3. A comparison of the effects of temperature and substrate concentration on the enzyme activity was made among these helminths. However, no definite relationship among them has been detected. 4. The significance of the existence of this enzyme in the helminths was briefly discussed.