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Notes: The fine structure and motility of spermatozoa and the composition of the seminal plasma of the perch Perca fluviatilis are investigated by electron microscopy, computer assisted cell motility analysis (CMA) and biochemical methods. The spermatozoon is asymmetrical as the flagellum inserts mediolateral on the nucleus. It lacks an acrosome, has an ovoid head and a small midpiece with one mitochondrion. Sperm motility–initiated in distilled water (10° C)–is characterized as follows: 85·0 ± 2·7% of the spermatozoa are motile, the main swimming type (10 ± 1 s after motility initiation) is the linear motion (61·4 ± 24·4%) and the average swimming velocity is 122·4 ± 21·9 μm s–1. When motility is initiated with NaCl, glucose or sucrose solutions of 100 mosmol kg–1 the percentage of motile spermatozoa and the swimming types are similar as in water, but the swimming velocity (174·0 ± 22·3 μm s–1) is significantly higher. Motility is inhibited by high osmolality of the diluent: when increasing the osmolality of the saline solutions to 350 mosmol kg–1 sperm motility is totally suppressed while potassium (10–40 mmol 1–1) does not affect motility parameters. pH optimum for sperm motility is between pH 7·0 and 8·5. The seminal fluid contains 124·01 ± 21·68 mmol 1–1 sodium, 10·22 ± 1·11 mmol 1–1 potassium and 0·72 ± 0·26 mmol 1–1 calcium. pH is 8·25 ± 0·09, and osmolality 283·90 ± 37·19 mosmol kg–1. The following organic components were determined: monosaccharides (glucose 63 ± 19 μmol 1–1, fructose 54 ± 28 μmol 1–1, galactose 59 ± 25 μmol 1–1), lipids (cholesterol 5·51 ± 6·42 μmol 1–1, triglycerides 72 ± l00 μmol l–1, cholesteryloleate 15–150 μmol 1–1, phosphatidylcholine 26 · 31 μmol 1–1, glycolipids 1–10 mg 100 m1–1), lactate 108 ± 99 μmol 1–1, hydroxybutyrate 102 ± 99 nmol 1–1, choline 59 ± 159 μmol 1–1, protein 344·75 ± 59·06 mg 100m1–1, enzymes (β-d-glucuronidase l.4 ± 0.7 μmol h–1 100 ml–1, protease (caseolytic activity) 1·0 ± 0·6 μmol h–1 100 ml–1, alkaline phosphatase 2520·0 ± 861·0 μmol h–1 100 ml–1, acid phosphatase 44.0 ± 16.0 μmol h–1 100 ml–1, glucose-6-phosphate dehydrogenase 38·9 ± 86·9 μmol h–1 100 ml–1, lactate dehydrogenase 134·4 ± 69·6 μmol h–1 100 ml–1, butyrylcholine esterase 0·014 ± 0·010 μmol h–1 100 ml–1, adenosine triphosphatase 562·8 ± 665·4 μmol h –1 100 ml–1).