Morphological, biochemical and minisatellite DNA variation was investigated at the colour ringed Brackenhurst House Sparrow population. Measurements and blood samples were collected from 584 nestlings and 692 other birds between 1985 and 1989. Six loci (6PGD, IDH, PEPD2, PEPD3, PEPT and transferrin) which had been the subject of a previous report (Burke, 1984) were investigated by starch gel electrophoresis. All followed Mendelian inheritance patterns, were in Hardy Weinberg equilibria and displayed temporal stability in allele frequencies. No evidence was found of the previously reported segregation distortion at PEPD3 and transferrin but artefact bands were encountered when scoring the latter. Family groups identified by observing colour ringed adults during feeding visits were examined using both enzyme and minisatellite DNA markers. Z chromosome linkage of several fingerprint bands was implicated, though most segregate independently. The probability of detecting an extra-pair fertilization was estimated as 0.5454 using starch gels and 0.9998 by fingerprinting. 51 out of 420 nestlings from 144 broods possessed several bands absent from the attendants' fingerprints. All nestlings with multiple mismatches shared many bands with the attendant female but a number consistent with band sharing between unrelateds with the male, i.e. nonparentage, was the result of cuckoldry. 24% of broods and 37% of males were affected. A correlation between the presence of extra-pair offspring and poor hatching success was noted. Cuckoldry was twice as successful in broods which contained infertile eggs. Metric variation was examined in the confirmed families. Significant heritabilities were demonstrated for weight, tarsus and tail length but environmentally induced variance was considerable. Yearlings were smaller than full adults in plumage length. This may be due to levels of protein reserves at critical growth periods. Some evidence of assortative mating for tail length was found which was unrelated to age associated changes.