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Slides were stained for 5':' 15min in 10% Giemsa (Gurr. improved R66) prepared in MilS phosphate buffer at pH 6.8. Stain was always freshly-made and filtered before use>~ Stained slides were rinsed with c1istilled water, blotted with bibulous paper, air:"dried,and placed in xylene for at least 15 min. Slides were' mounted in Permount' with a No.1 coverslip.' Fi ni shed s 1 i des were observed, us ing a Ze iss Photomi croscope 11 equi pped With a 100 W mercury vapor lamp. Photographs were taken through a 100X phase objective with Kodak Technical Pan 2415 at ASA 50 and a green substage filter. Film was developed in Kodak 019 solution and printed on Kodak Paper using Dekto1 developer. After preliminary sorting of chromosomes from those photo- graphs in which chromosomes were neither overlapping nor excessively con-_ tracted, each chromosome was measured. The quantitative characterization involved measurements of the relative length (expressed as 0/00 total length of the mitotic chromosome complement) and the arm ratio (short arm I long arm X 100). All measurements were taken with calipers and a ruler graduated in 1/60 inch units. / There are sites on the chromosome whose function is to make components for the cellular entity that manufactures proteins. These are called nucleo- lar organizing regions (NORs), for which interpopulational differences have been noted in higher vertebrates (Ruiz et a1., 1982). In order to stain for NORs, the procedure described above for Giemsa-staining was followed with certain changes. Gill arches were fixed for no longer than 30 min in two changes of fixati~e. Slides were made immediately, using the standard pro- cedure at 37 - 40 C, and stored in a cool, dry location for no more than 48 hours before further processing. Silver stain was freshly prepared by mixing I g silver nitrate in 1% formalin (one part formaldehyde solution added to 35 parts of distilled water). A few drops of freshly prepared stain are added to each slide. After applying covers 1 ips, the sl ides were placed in a moist chamber at 370 C for 4 - 8 hours. Finished slides were rinsed in distilled water and counterstained in 10% Giemsa (Har1eco) in MilS phosphate buffer at pH 6.8 for no more than 30 seconds. Dried slides were soaked in xylene for 30 min and mounted in Permount. Kodak Panatomic X fi 1m at ASA 32 was used for photographing silver:stained spreads. Film was developed in Microdol. Other- wise, the quantitative characterization for Giemsa-staining was followed. RESULTS All fishes were found to have 25 pairs of chromosomes in each mitotic cell. Since measurements were taken from a large number of Giemsa-stained chromosome spreads.. (n= 40) from e,ach population sample, a detailed quanti...; fi cation, of chromoso~ length" and centromere placement was made ". (Table l;,:'r ~ remaining dataavailableuponreCJuest).',? Most.chromosomes appear simila~be";' - tween populations and are not discussed further in this report. There is' an ,~, , i .~~