A quantitative measure for assessing the level of resistance of zebu cattle to Rhipicephalas appendiculatus is discussed. Basic data for the derivation of the mathematical index were from artificial infestation of cattle to adults, nymphs and larvae ofthe Rusinga Island stock of the tick species. Observed variables were the weights and numbers of the engorged/detached instars.

The index derivation process was by the use of the stepwise discriminant function procedure and the Mahalanobis distance. These procedures led to the derivation of an index which was a function of the engorged weights of the female and nymphal instars, the numbers of detached females and the Mahalanobis distance. Variables associated with the larval instar were found to be insignificant and were therefore not included in the index function.

The derived index is capable of discriminating between susceptible and resistant cattle, and can be used to monitor changes in the resistance levels of cattle over time.

Un index mathematique base sur les resultats obtenus aprésune infestation des vaches avec le Rhipicephalus appendiculatus a été etudle. L'index est derive d'une fonction analytique discriminante qui a comme principle variables le nombre des femelles ayant pris du sang et les polds des femelles et chrysalides engorgeés. Cet index est capable de discruninér entre les vaches sucepttbles et resistantes et peut être utilises pour observer les changements du niveau de resistance chez les vaches.