Abstract — Plant diseases are among the major challenges faced by the oil palm cultivation in Nigeria. The major plantation diseases of the oil palm include Fusarium wilt (F. oxysporum fsp. elaeidis), dry basal rot (Ceratocystis paradoxa), Ganoderma trunk and recently neck bending/inclination of the upper region of the palm noticed in some Southern and Western States of Nigeria. Pathological and molecular investigation into the etiology of the disease was found to be caused by Thielaviopsis ethacetica. Depending on the disease severity on the oil palm, the symptoms include gradual inclination of the palm, yellowing of fronds, drying of the leaves of the palm from the base, production of a fermented fruit odor, and finally the collapse and death of the palm. T. ethacetica was consistently isolated from the diseased oil palms; produced numerous spores of two types (microconidia and macroconidia) and were further characterized using spores sizes, morphology and sexual compatibility test. The aggressive nature of T. ethacetica on the oil palm was confirmed by the sexual compatibility test resulting in the presence of sexual spores in the tissues of the severely affected palms while asexual spores were present in tissues of the mildly affected palms. The outcome of this study confirmed T. ethacetica to be the causal agent of neck bending/inclination of the upper region of the oil palm in Nigeria.