Potassium fertility experiments were observed in 1993 to measure possible differences in twospotted spider mite (Tetranychus urticae Koch) infestation. Spider mite counts showed lower infestation levels in all deep placement potassium rates, compared to untreated plots which were potassium deficient. Cotton (Gossypium hirsutum L.) variety was DES 119. The highest rate of 720 lbs K₂O/A resulted in a significant reduction of 45% compared to no potassium treatment. Subsequent observations of twospotted spider mite infestations were made in 1995 and 1997 in a multiple year study of potassium fertilization effects on eight cotton varieties. Varieties in the test were Stoneville 132, Hartz 1215, SureGrow 125, Hartz 1330, Deltapine 50, SureGrow 501, Stoneville LA 887, and Deltapine 5415. The potassium fertilizer treatment by cotton variety interaction was not significant for 1995 data but was highly significant for 1997 data. Varieties on which twospotted spider mites were significantly lower in two or three potassium treatment rates than in the no potassium treatment were Deltapine 5415 and Hartz 1215 in 1995, and SureGrow 125, Hartz 1215, and Hartz 1330 in 1997. Potassium fertilization of cotton showed significant reduction of twospotted spider mites in cotton with a strong varietal influence. Spider mite infestation level was significantly lower in potassium treatments in some varieties while little or no effect was seen in other varieties. Varieties in the study observed in 1995 and 1997 represented a wide range of maturity characteristics from very early to late maturity. Potassium fertility effect on spider mites did not appear to be related to the maturity characteristic of the cotton varieties.