The objective of this thesis is to examine what influences the distribution of different sized mesozooplankton at the mesoscale. Zooplankton distributions are influenced by both the physico-chemical environment and by biological processes. To study the relationship between these processes at the mesoscale, biological and physical variables were measured concurrently using a combination of the SeaSoar, fluorimeter, optical plankton counter (OPC) and EK500 echosounder instruments. These data were collected in March 1997, during a survey of the Strait of Hormuz, that connects the Arabian Gulf to the Gulf of Oman in the North West Indian Ocean. OPC data were used to describe and study the distribution of mesozooplankton. The accuracy of the OPC's estimates of abundance and biovolume was determined by comparison with net samples of zooplankton collected by a Longhurst-Hardy Plankton Recorder (LHPR). The correlation of the OPC abundance data with the net samples showed an approximate 1:1 relationship, suggesting that the OPC was capable of reliably enumerating mesozooplankton. Biovolume was first calculated from the sizes of particles measured by the OPC using a spherical model for the zooplankton. The resulting OPC biovolume data were consistently larger than the biovolume measured in the net samples. The OPC biovolume was then calibrated using a spheroidal model, which better represented the zooplankton in the Strait and incorporated measurements of the dimensions of animals in the net samples. The resulting OPC biovolume was within the range of the net samples, although the OPC overestimated biovolume relative to the net at low in situ concentration.