This study introduces modified method of ESEM for observing representatives of the selected groups of helminth parasites from fish and spider. The most suitable treatment methods and optimal environmental condition for observing selected parasites were found. Already fixed samples (70% ethanol) were washed and rinsed with distilled water. Observations were made on a non-commercial ESEM AQUASEM II [6]. Samples were taken to the Peltier cooled specimen holder with silicon surface, into a drop of water. Observation conditions were: beam accelerating voltage 20 kV, probe current 110 pA, sample temperature 2 °C, water vapour pressure 650-700 Pa. Here, we demonstrated this method on the fish parasitic helminths and also on the subadult of mermithid parasite (Nematoda: Mermithidae). It was found parasitizing Caerostris sumatrana (Araneae: Araneidae), orbweb species of spider living in shrubs and trees of lowland dipterocarp forests. The finding was made in Phetchaburi province (Ban Lat district) in south part of Central Thailand. This is a first record of mermithid parasite from orb web spider species living in higher vegetation. The surface was imaged in wet state without covering water layer to reveal microstructural specifics of samples. "In-situ" observation of non-conductive biological-parasitological samples was free of charging artefacts and the visibility of topographical structures was sufficient for taxonomical research. Best displaying results were obtained at Nematoda and proboscis of Acanthocephala. The suitability of ESEM for study of parasites in wet state and free of any preparation was proved by our results.