Hardlock is a cotton disease characterized by the increase in density of cotton boll fibers. These compacted bolls are difficult to harvest, resulting in considerable losses to growers. Yield is usually reduced by 20 to 60% by the end of the season if hardlock is present. Fusarium verticillioides is the most common fungus isolated from affected plant parts. Utilizing PCR based methods; we have sequenced the internal transcribed spacer (ITS) region of the Fusarium isolates T2, T5 and T6. ITS sequence analysis demonstrated that isolate T2 is F. verticillioides and T5 and T6 are F. proliferatum.  Morphological analysis of spores also demonstrated the similarities between T5 and T6 isolates. In cottonseed-agar, F. verticillioides and F. proliferatum produce around 1% and 10 % of macroconidia among the conidia population, respectively. We are currently evaluating the involvement of each of these species in hardlock. The combined inoculation of the two species is also being tried. A positive correlation was found between seed weight and fiber quality. Healthy seeds always exhibit more dry weight than diseased cottonseeds. Low cottonseed weight is associated with severely hardlocked cotton bolls. These results suggest that the casual agent of hardlock is consuming lipid cottonseed reserves, since there is a decrease in oil reserves with the disease. Fumonisins (B1, B2 and B3) were assessed. Fumonisins are not easily produced in cottonseed-based media; however, it was detected in isolates growing on PDA plates. We hypothesize that cottonseed oil or its byproducts may suppress toxin production. Dark field and phase contrast microscopy have been used to monitor symptom development. Microscopic observations revealed fungal debris, hyphae involving cotton fibers and Fusarium-like macroconidia. This is the first direct observation of Fusarium-like conidia in affected tissues.  The progress of this work may unequivocally place Fusarium sp. as the causal pathogen for cotton hardlock.