Aims: To study the kinetic passage of Lactobacillus plantarum 44a from feed to faeces of tilapia in order to calculate the number of Lactobacillus excreted. Methods and Results: In a single-dose experiment, duplicate lots of 26 fish devoid of intestinal lactobacilli were fed with diets containing c. 4 center dot 5 x 1011, 6 center dot 3 x 108, 6 center dot 0 x 105 and 0 CFU of Lact. plantarum 44a per single feed ration. In the multiple-dose experiment, duplicate lots of 30 fish each were supplied with a diet containing 1 x l011 CFU of Lact. plantarum 44a as follows: 14 times in 14 days, five times in 14 days, once in 14 days and zero times in 14 days. Faeces were periodically collected and analysed for their lactobacilli content by using a selective media. The kinetics of Lactobacillus in the faeces was described as a pulse signal defined by three parameters: theta, A(o) and the time. theta, was identified as the time to reach the peak (x axis) and A(o) was a constant. A(o) divided by e, was identified as the height of the peak (y axis). The area below the curve A(o)theta allowed the calculation of the total number of lactobacilli excreted. The ability of the mathematical model to describe the actual values was tested by a linear regression analysis. In most of the cases, the equations showed an intercept close to zero (P > 0 center dot 05) and angular coefficients near one. Conclusions: Lactobacillus plantarum 44a was excreted in short pulse signals described by a mathematical model which allowed calculating the area below the curve and consequently the survival rate. Significance and Impact of the Study: This study provides a quantitative method to study the kinetics of excretion of a probiotic bacterium in the faeces.