The work described is primarily concerned with measuring the induced AC electrokinetic properties of the transmissive stages of two water-and food-borne intestinal pathogens, namely oocysts of Cyclospora and eggs of Ascaris, in order to demonstrate the applicability of electrorotation for determining the viability of the organisms. An electrode design that maximises the space over which a uniform electric field is applied to the electrorotation chamber is described. This is important when investigating organisms that are few in number, or for investigating large, mixed populations. It was found that Cyclospora viability and sporulation state could be determined using electrorotation. Since there is no vital dye assay for this emerging parasite, and its life-cycle is not fully understood, these are important findings for the water and food industries. Modelling of the electrorotational response was found to be in good agreement with the experimental data. Limited electrorotational spectra from oocysts recovered from baboons showed slight differences when compared with those of human source. The first electrorotational studies of a complex biological organism, namely the eggs of the nematode Ascaris, which poses a global health problem, are presented. It was found that unfertilised and fertilised eggs rotate in opposite directions, allowing for rapid visual determination of mixed populations. Different stages in the development of fertilised Ascaris eggs were also observed by electrorotation. Modelling of the electrorotational response of the Ascaris eggs was found to be in reasonable agreement with the experimental data. The work presented here shows that the electrorotation technique can be used for the rapid determination of the viability and sporulation state of oocysts of C. cayetanensis, and the fertilisation state of eggs of A. suum. As current viability assays are complicated and time consuming, requiring trained personnel, the electrorotation technique offers many advantages to the food and water industries, especially as it can be automated.