[A description is given of the courtship of the viviparous cyprinodontid fish Lebistes reticulatus and an ethological analysis is attempted of the behaviour of the male during these performances. Several courting activities are distinguished. They most frequently occur in the order: searching, approaching the ♀, following, posturing, luring, sigmoid, this series ending either by a display jump or by a copulation attempt. This sequence corresponds to an increase of the intensity of the sexual drive; the order is not rigidly fixed, it is for instance to a considerable extent influenced by changes in the external situation. The courtship consists of two phases: 1°. a preliminary one in which the ♂ tries to lead the ♀ away from a concentration of guppies, involving the series of activities already mentioned and ending with a display jump; 2°. a more advanced stage, including the same activities but ending with a copulation attempt. In correspondence with the function in the first phase the body axes of ♂ and ♀ are aligned with each other but during the second phase (which we called checking) the ♂ halts the advance of the ♀ by placing its body axis perpendicularly to hers. By making genital smears we could distinguish copulation attempts during which real copulation took place from merely superficial gonopodial contacts. True copulations appeared to be always followed by a characteristic activity: the post copulatory jerking. The material we used was far from a pure breed and consisted of mixtures of different races, at least of iridescens, aureus, oculatus and variabilis. Therefore, the marking pattern in the (non-reproductive) ♂.♂ varied considerably from fish to fish. However, during courtship, the ♂ ♂ we studied assumed the same pattern of black markings, whereas at the same time most of the existing individual differences faded away. Six different groups of markings were distinguished during courtship, they were named 1, 2, 3, 4, 5 and 6. Quantitative studies of the occurrence of these patterns show that 3 and 5 are the only patterns occurring exclusively in combination with another pattern, namely 3 with 2 and 5 either with 4 or 6, whereas 1, 2, 4 and 6 can all occur alone. Moreover, pattern 1 can occur with 2+3, 4 or 6; 2 can be combined with 4; combinations of 2 and 6 and of 4 and 6, however, are rare and if present are incompletely developed. It is suggested that the different colour patterns correspond to different internal factors or combinations of internal factors. In that case the simultaneous presence of 2 and 6 or of 4 and 6 must indicate the contemporary influence of two internal factors that are in principal incompatible with each other. The order of the different combinations of black markings during courtship was also studied quantitatively. The results suggest that after a preliminary phase in which pattern 1 is predominate a phase follows in which pattern 4 dominates. In between both phases pattern 2 or combinations of 2 and 4 are often seen. After a longer or shorter period of courtship the 4-phase is succeeded by a 6-phase. Further quantitative studies were made to find out how often every activity is combined with every combination of marking patterns. These figures indicate that pattern 2 corresponds with aggressive behaviour, both 4 and 6 with courting behaviour, which reaches the highest level at 4.5 and 6.5. Successful copulations marked by post-copulatory jerking are, however, only performed when 6.5 is shown. This pattern, therefore, must correspond to the highest level of the sexual motivation. The sigmoid display is seen with the 4- as well as with the 6-pattern; however, the sigmoid activity in both phases shows the above mentioned difference in orientation. In the preliminary phase with the 4-pattern the sigmoid posture is directed away from the ♀ and in the advanced phase with the 6-pattern it is displayed perpendicularly to the body axis of the ♀, the orientation of the activities and the position of the patches, therefore, work together to make the latter very conspicuous to the ♀. Obviously the fixed motor pattern (