The biological control of helminth parasites of bovines by nematophagous fungi is an alternative to the use of drugs with the principal objective of reducing the source of infection available on pastureland. The maintenance of predatory activity of the fungal isolates is one of the basic prerequisites to ensure the success of this form of control. In this study behaviour of the isolates I31 of Arthrobotrys robusta and NF34a of Monacrosporium thaumasium was investigated following three storage methods: stored at 4 °C, cryopreserved with or without cryoprotectants or preserved in silica gel. All samples were subsequently passed through the gastrointestinal tract of calves. The latter involved the administration of 20 g of mycelia to the animals. This quantity was sufficient to recover fungal material from the faeces. The peak reduction in the number of infective larvae in the faeces occurred 24 h after administration of the samples (P < 0.05). The storage at 4 °C was the treatment that produced the greatest reduction in larvae for NF34a (81.3%) and I31 (65.1%) isolates. Nf34a isolate was responsible for the highest percentage reduction of larval helminth populations (P < 0.05). Cryopreservation appears to be an efficient method of preserving isolates, although diminished predatory capacity compared to storage at 4 °C was seen only for isolate NF34a (73.2%). Cryopreservation did not interfere in predatory activity of I31 isolate (P < 0.05). Maintenance of isolates in silica gel showed the lowest reduction throughout the experiment (P < 0.05).