The study aimed to determine the level of knowledge of farmers' respondents aboutpractices rationalization of chemical pesticide use in Tomato pests resistance, determine the relationship between the level of knowledge of farmers' respondents related to those practices, determine the percentage of the contribution rate of some independent variables to explain the variance in the dependent variable, also determine the information resources which farmers take their information about practices rationalization of chemical pesticide use, finally the role of agricultural extension in the definition of farmers about those practices .    The research conduct in the largest three villages of the largest three districts  growing Tomato in El Minia governorate, during was selected sample farmers using the equation Krejcie and Morgan reached 210 farmers which representing about 45.6% of the population of tomato farmers which had reached about 460 farmers ,  respondents were selected randomly from the tenure records of the agricultural cooperation organization in selected villages during 2015, data were collected using a prepared personal questionnaire during .October 2015. Frequencies, percentages, mean, standard deviation, simple correlation coefficient of Person, chi square, multiple regression analysis step–wise used to analyze the studied data. The most important results of the study include: 1– The level of knowledge of farmers' respondents aboutpractices rationalization of chemical pesticide use in Tomato pests resistance was moderate 68.6%. 2– There was a significant relationship between the degree of knowledge of farmers' respondents aboutpractices rationalization of chemical pesticide use in Tomato pests resistance and each of following independent variables: number of education years, experience in tomato, average production per faddan, the geographical openness, self-assessment of opinion, a full –time agricultural work.  3–There are three independent variables collective explain 11.1% of the total variance in dependent variable. 4– Friends, dealers' pesticides, neighbors have the priority as sources of information to the farmers as they mentioned. 5–Agricultural extension role was low in the definition of farmers aboutpractices rationalization of chemical pesticide use in Tomato pests resistance.