Abstract: Age determination of Citharinus citharus (Pisces: Citharinidae) and Synodontis nigrita (Pisces: Mochokidae ) in Lake Ona, Southern Nigeria were made in a study that spanned 2001 to 2003. The fish species were caught, using varied fishing gears that included:- bottom set and surface set gill nets of mesh sizes 1.0, 3.0 and 5.0 cm , with each having a length of 25.0 m and a depth of 3.0 m; cast net with stretched mesh size of 6.4 cm; fish baskets, non-return valve traps which along with fences formed fish pots and a set of long-lines of length 35.0 m. Fish Aggregating Devices (FAD) were deployed to aggregate fishes and invariably increased fishing success. Sampling was at three Stations (I, III and V) established on the Lake. In the Laboratory, specimens collected were identified and sexed, their total and standard lengths were measured in centimeters while their and body weights were taken in grams. The age of fish species were determined by analysis of Frequency distribution of Ocular Lens Diameters (OLD) Measured in Millimeters (mm) and validated by means of Length-Weight frequency distribution. Catch statistics were, 288 specimens of S. nigrita with a sex ratio of 1.6:1,TL range of 04.5-18.0 cm and BW range of 05.5-56.3 g ; 308 specimens of C. citharus with sex ratio of 1.4: 1, TL range of 06.0-21.5 cm and BW range of 05.8-581.2 g. Results of the Frequency distributions of OLD in S. nigrita studied indicate the population was made up of one age group with modal Class interval of 10.0-12.5 cm. In C. citharus, a single age group was clearly evident with modal Class interval of 15.0-17.5 cm. A lone Class interval of 25.0-27.5 cm, distal from the bulk may suggest another age group of C. citharus not fully represented in the sample. Length and Weight Frequency distributions of both species equally followed the same pattern as the OLD frequency distribution confirming mainly single age groups in the studied populations. Results obtained equally indicate there was a significant difference (p<0.05) between the left and right OLD in both species irrespective of sex. In C. .citharus, all the left lenses were bigger than the right ones. In S. nigrita however, 253 (88.77%) of the population had bigger left OLD while the remaining 32 (11.23%) had the right OLD bigger than the left ones. Since all other features used in identifying the specimens were the same, except for the observed differences in the eye lens of S. nigrita, it is suggestive that we may have seen the outset of speciation.