Population density in four study areas for P.guttatus was 11-34 animals in the exploited phase/hectare (ha.) from capture-recapture work. The population densities of P.guttatus for its major physiographic environments were established from capture-recapture and transect work. Greatest densities were on the Atoll-Rim and Main (10 fa.) Terrace, (34-39 animals/ha.). Lowest were on the Reef Tract's patch-reef zone (2-3 /ha.). Present size at first physical maturity of P.argus females was 81-82 mm carapace length (C.L.), from leg length index (L.L.I.) measurements and 80-83 mm C.L. from presence of eggs and/or spermatophores, and for males was 96 mm C.L. from L.L.I. analyses; P.guttatus female maturity was at 53-58 mm C.L. from C.L./tot. length of male and female. The Von Bertalanffy growth parameters for P.argus and P.guttatus, at Bermuda were: K = 0.18 and 0.15 for male and female P.argus respectively; Similarly, K = 0.22 and 0.155 for P.guttatus; L ∞ = 204 mm and 192 mm C.L. for male and female P.argus respectively; Similarly, L ∞ = 87.6 mm and 86.0 mm C.L. for P.guttatus; t₀ = - 0.82 years and - 1.0 years for male and female P.argus respectively; Similarly, t₀ = - 0.846 yrs, and - 1.12 yrs, for P.guttatus. Details of the biological parameters for lobsters of the Northern Fishing Grounds were established: (1) Male P.argus; Z = 1.60, E = 0.72, F = 1.15, M = 0.45; 2) Female P.argus; Z= 0.99, E = 0.72, F = 0.71, M = 0.28; 3) Male P.guttatus; Z = 0.81, E = 0.50, F = 0.40, M = 0.41; 4) Female P.guttatus; Z = 0.77, E = 0.66, F = 0.51, M = 0.26. Parameters for Argus Bank were: (5) Male P.argus: Z = 0.52, E = 0.41, F = 0.21, M = 0.31; (6) Female P.argus: Z = 0.32, E = 0.41, F = 0.13, M = 0.19, (Z, annual total mortality coefficient; E, exploitation rate; F, M, annual fishing & natural mortality coefficients). Unknown facets of the field behaviour of P.guttatus were revealed and a further insight into its ecology was gained. Knowledge of the field behavior of both juvenile and adult P.argus was advanced. More than 64 juvenile P.argus chiefly of size 20.0-37.5 mm C.L. were attracted to haulable artificial shelters casitas with built-in predator-barriers, set as a group with spacing of 20 metres, inshore in seagrass; lobster habitat was enhanced. Most (ca. 70%) of the eggs produced by P.argus at Argus Bank were laid by females of C.L. > 126 mm C.L. Trapped sex ratio of P.guttatus was 116 females: 1,866 males. Legal/sublegal ratio for P.argus was 1.15±0.176. The annual fishing mortality coefficient of P.argus for Bermuda Platform was 0.34 (from tag returns) and the reciprocal of the catchability coefficient 134,114±2421 (standard error). The number and density of P.argus post-recruits in the reef areas of Bermuda Platform was 163,000 animals and 3.7 animals/ha. or 407 kg/sq.km. (standard error of 7.5%), the median from stock assessment and tag returns. Density of P.argus on the Atoll Rim at Kitchen Shoals was 6.4/ha. and on the Easter Main Terrace 7.8/ha. from capture-recapture and transect work. Annual mean population numbers and annual equilibrium catches of P.argus (of > 92 mm C.L.) were estimates for the seasons 1975/76 to 1986/87: since 1975 there has been a trend of slowly increased annual mean population number or fluctuation around a stabilised level of about 75,000 legal-size lobsters. M.S.Y. was 31,800 lobsters/season. Optimum fishing effort was 78,600 hauls/season. Standing stock of P.argus at Argus Bank was 5,000 kg (3,000 legal lobsters). Density at Argus Bank was ca. 175 kg legal-size stock/sq.km. (1 legal animal/ha.): potential yield was 1 tonne/cal. yr. (600 animals/cal. yr.).