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Notes: Summary 1. Pollimyrus isidori displays two types of electric organ discharge (EOD) phase-locking behaviours with EODs of other individuals, a given individual always displaying the same type. The preferred latency response (PLR) maintains preference of an EOD interval of 10–15 ms with another individual's EODs; the other type is the preferred latency avoidance (PLA) which consists of an avoidance of 10–20 ms intervals with another individual's EODs. We investigated whether these two EOD phase-coupling behaviours are age- or sex-related. 2. A morphological sex characteristic reported in various mormyrid genera was also found to be present in P. isidori. The base line of the anal fin of females is straight or slightly convex, while it is slightly concave in males (Fig. 2). The animals were sexually mature but not ready to spawn. Sex was determined from gonad histology (9 males, 15 females; Fig. 6). 3. No conspicuous sexual dimorphism in EOD wave form was found (Fig. 3). Morphologically recognizable males (n=11) display PLR, while morphologically recognizable females (n=12) show PLA (Fig. 4). In six of the males and four of the females sex was also confirmed histologically. Young fish (n=6) below a critical range of 5.7–6.3 cm body length show random EOD latency distributions to the stimulus pulses (Fig. 4C); the sexes of these fish could not be determined by the morphological criterion. Only when they had grown to body lengths beyond the critical range did each individual display either PLR (n=4) or the PLA (n=2; Fig. 5). The individuals did not change the type of response (followed up to 20 months). The emergence of the sex difference in electrical behaviour proceded considerably the emergence of the morphological sex difference of the anal fin beginning at 7.4 cm body length. 4. For the following reasons, it is suggested that PLR or PLA is not a jamming avoidance device: (1) neither type of EOD latency behaviour effectively reduces the probability of EOD coincidences below what was expected from random occurrence; (2) the probability of EOD coincidences in two independently discharging fish is exceedingly low (1–3 coincidences in 1,000 pairs of pulses) as a result of low repetition rates (10–25 EODs/s) and short EOD duration (50 μs).