Ever since Falconer (1960) discussed how additivegenetic variance for fitness rapidly decreases both theoreticaland empirical interpretations have shown that there is a lowparent-offspring correlation for fitness (Maynard-Smith,1978). Williams (1975) discussel with his characteristicintellectual power that under equilibrium genetic variationaffecting fitness should not be heritable. Williams’ Sisypheangenotype terminology (instead of Dobzhansky’s (1964a)genetic elite) signifies that low heritability of reproductivesuccess or viability would be instrumental in dropping themost fit genotypes down into the range of mediocrity in thenext generation. Thus, a female does not produce fitteroffspring by mating with an above average male (Maynard-Smith, 1978) or by mating several times (Bateman, 1948).More recently other reportes (Partridge, 1980) have foundthat when females choose there mate, their offspring aremore successfully competitive. Our results (Table I) partiallyagree with those of Partridge.Females from the progeny of a mass culture withthree original isofemale lines (10, 11 and 41 freshlybrought from nature) were placed to oviposit and to growprogeny. From an abundant progeny, three randomlypicked groups (A, B, and C) were established in vials.Group A consisted of ten virgin females, randomly chosenfrom the original mass culture mentioned above, crossedto a single male, also randomly chosen out of isofemaleline 10 from Oicata, Colombia. Group B consisted of avirgin female, randomly chosen from the same originalmass culture, and a single male, also randomly chosenfrom males of the isofemale line 10 made into a massculture. In group C, groups of 10 pregnant females fromthe mass culture were allowed to oviposit in vials. Threedifferent comparisons were made. The offspring of groupA compared with the offspring of group C was A, C. Thecomparison was made by taking first instar larvae fromeggs laid by females from group A (no choice, tenfemales) and first instar larvae from eggs laid by femalesfrom group C (choice, 10 females) and competing eachset of larvae against and equal number of larvae bearingthe recessive mutation vermilion (eye color) from a D.pseudoobscura stock made exclusively for this experiment.In group B, the offspring of a single adult virginfemale taken from the mass culture from Oicata and matedsingly to a male, also taken at random from the isofemalestock 10, were compared with offspring of inseminatedfemales collected from the mass culture (this is B, C). Aswith A, C, comparison B, C, was made by taking first ins-tar larvae from eggs laid by females from group B (nochoice, one female) and first instar larvae from eggs laidby females from group C and competing each set of larvaeagainst an equal number of larvae bearing the recessivemutation vermilion.The offspring of group A, compared with theoffspring of group B was A, B. The comparison was madeby taking first instar larvae laid by females from group A(no choice, 10 females) and first instar larvae from eggslaid by females from group B (no choice, one female) andcompeting each set of larvae against an equal number oflarvae bearing the recessive mutation vermilion from a D.pseudoobscura stock.Each group (A, B and C) consisted of 10 replicates.Two experiments were performed, one with 400 larvae onewith 300 larvae. In the 400-larva experiment each set of200 larvae from each group A, B and C were placed with200 larvae of the recessive mutant vermilion. For the 300-larva experiment, each set of 150 larvae from each groupA, B and C were placed with 150 larvae of the recessivemutant vermilion.Larvae were allowed to compete in plastic vials,which contained the same food medium (banana-agar-propionic acid medium) as that used in the mass culturefrom Oicata. The number of wild type and vermilion fliesemerging from each vial was recorded. We endeavored tohave the larvae compete under conditions similar to thoseencountered by larvae in the original mass culture fromOicata. As far as we know food, temperature, CO