The repertoires of physiological responses which invertebrates possess play a major role in determining their distribution and survival potential. In the present study the effect of severe hypoxia on the heart rate of S. entomon has been investigated. Saduria entomon are often exposed to extended periods of severe hypoxia and anoxia in their natural habitats. The animals were exposed to conditions of a reduced level of oxygen ( 10% O2 saturation) for 24 hours. The experiment confirms that the pattern of their heart rate changed considerably with time and experimental condition. Under hypoxic condition a steady, regular heart rate was observed and the variability of beat rate decreased in comparison with normoxic one. The mean heart rate of the unburied animals in normoxia was higher than that of the buried S.entomon with considerably lower inter-individual variability. The mean heart rate of animals in hypoxic conditions was similar for all individuals (41.66 ą 2.03 (mean ą SD)), but not significantly different in normoxic conditions (Student t-test, p 0.05).