An in situ system consisting of a mini-rhizotron in conjunction with a micro-video camera attached to a color monitor or VCR has been developed to quantitate hyphal strand development and subsequent sclerotium formation of Phymatotrichum omnivorum with cotton as a host. Soil water potential was monitored with thermocouple psychrometers and tensiometers. Each mini-rhizotron consists of an acrylic tube 5 cm in diameter x 60 cm in length, etched with 1.5 x 1.5 cm grid to permit video recording of fungal development. Hyphal strand development and cotton roots associated with P.omnivorum strands were quantified with Newman's grid-intersect method. Strand development adjacent to the mini-rhizotron appeared linear when expressed as total strand length. Total strand growth was greater towards the soil surface than down the soil profile during the first 14 days of the experiment when sclerotia were introduced as inoculum at a depth of 30 cm. After 28 days strand growth equilibrated throughout the soil profile.